Address compilation warnings

[hpoettker]

Compiling the code with

pip install -v .

on Ubuntu produces a lot of warnings. Most of them come from ReadStat but not all of them.

Below, I'll post the relevant section of the output of

pip install -v . 2>&1 | grep -A3 pyreadstat

This PR addresses these particular issues. Most of the warnings are about missing const qualifiers but there are also two potentially uninitialized variables and an unused function.

  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_convert_readstat_to_python_value’:
  pyreadstat/_readstat_parser.c:7739:25: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   7739 |     __pyx_v_c_str_value = readstat_string_value(__pyx_v_value);
        |                         ^
  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_handle_metadata’:
  pyreadstat/_readstat_parser.c:8608:24: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   8608 |   __pyx_v_mr_sets_orig = readstat_get_multiple_response_sets(__pyx_v_metadata);
        |                        ^
  pyreadstat/_readstat_parser.c:8899:23: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   8899 |   __pyx_v_flabel_orig = readstat_get_file_label(__pyx_v_metadata);
        |                       ^
  pyreadstat/_readstat_parser.c:8908:26: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   8908 |   __pyx_v_fencoding_orig = readstat_get_file_encoding(__pyx_v_metadata);
        |                          ^
  pyreadstat/_readstat_parser.c:9055:17: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   9055 |   __pyx_v_table = readstat_get_table_name(__pyx_v_metadata);
        |                 ^
  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_handle_variable’:
  pyreadstat/_readstat_parser.c:9250:20: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   9250 |   __pyx_v_var_name = readstat_variable_get_name(__pyx_v_variable);
        |                    ^
  pyreadstat/_readstat_parser.c:9621:21: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   9621 |   __pyx_v_var_label = readstat_variable_get_label(__pyx_v_variable);
        |                     ^
  pyreadstat/_readstat_parser.c:9691:22: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   9691 |   __pyx_v_var_format = readstat_variable_get_format(__pyx_v_variable);
        |                      ^
  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_handle_value_label’:
  pyreadstat/_readstat_parser.c:12029:27: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  12029 |       __pyx_v_c_str_value = readstat_string_value(__pyx_v_value);
        |                           ^
  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_check_exit_status’:
  pyreadstat/_readstat_parser.c:13268:26: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  13268 |     __pyx_v_err_readstat = readstat_error_message(__pyx_v_retcode);
        |                          ^
  pyreadstat/_readstat_parser.c: In function ‘__pyx_f_10pyreadstat_16_readstat_parser_run_readstat_parser’:
  pyreadstat/_readstat_parser.c:13941:5: warning: ‘__pyx_v_error’ may be used uninitialized [-Wmaybe-uninitialized]
  13941 |     __pyx_f_10pyreadstat_16_readstat_parser_check_exit_status(__pyx_v_error); if (unlikely(PyErr_Occurred())) __PYX_ERR(0, 1009, __pyx_L1_error)
        |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  pyreadstat/_readstat_parser.c:13359:20: note: ‘__pyx_v_error’ was declared here
  13359 |   readstat_error_t __pyx_v_error;
        |                    ^~~~~~~~~~~~~
--
  pyreadstat/_readstat_writer.c: In function ‘__pyx_f_10pyreadstat_16_readstat_writer__check_exit_status’:
  pyreadstat/_readstat_writer.c:11028:26: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  11028 |     __pyx_v_err_readstat = readstat_error_message(__pyx_v_retcode);
        |                          ^
  pyreadstat/_readstat_writer.c: At top level:
  pyreadstat/_readstat_writer.c:10997:13: warning: ‘__pyx_f_10pyreadstat_16_readstat_writer__check_exit_status’ defined but not used [-Wunused-function]
  10997 | static void __pyx_f_10pyreadstat_16_readstat_writer__check_exit_status(readstat_error_t __pyx_v_retcode) {
        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  pyreadstat/_readstat_writer.c: In function ‘__pyx_f_10pyreadstat_16_readstat_writer_convert_datetimelike_to_number’:
  pyreadstat/_readstat_writer.c:5016:20: warning: ‘__pyx_v_tstamp’ may be used uninitialized [-Wmaybe-uninitialized]
   5016 |     __pyx_v_tstamp = (__pyx_v_tstamp + __pyx_v_offset_secs);
        |     ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  pyreadstat/_readstat_writer.c:4850:10: note: ‘__pyx_v_tstamp’ was declared here
   4850 |   double __pyx_v_tstamp;
        |          ^~~~~~~~~~~~~~