Fix use of uninitialized value when parsing regexp

[peterzhu2118]

Parsing the regexp /\A{/ causes uses an uninitialized value because it tries to parse it as a range quantifier, so it reads the character after the closing curly bracket. This is using uninitialized values because prism strings are not null terminated. This can be seen in the Valgrind output:

==834710== Conditional jump or move depends on uninitialised value(s)
==834710==    at 0x5DA010: pm_regexp_parse_range_quantifier (regexp.c:163)
==834710==    by 0x5DA010: pm_regexp_parse_quantifier (regexp.c:243)
==834710==    by 0x5DAD69: pm_regexp_parse_expression (regexp.c:738)
==834710==    by 0x5DAD69: pm_regexp_parse_pattern (regexp.c:761)
==834710==    by 0x5DAD69: pm_regexp_parse (regexp.c:773)
==834710==    by 0x5A2EE7: parse_regular_expression_named_captures (prism.c:20886)
==834710==    by 0x5A2EE7: parse_expression_infix (prism.c:21388)
==834710==    by 0x5A5FA5: parse_expression (prism.c:21804)
==834710==    by 0x5A64F3: parse_statements (prism.c:13858)
==834710==    by 0x5A9730: parse_program (prism.c:22011)
==834710==    by 0x576F0D: parse_input_success_p (extension.c:1062)
==834710==    by 0x576F0D: parse_success_p (extension.c:1084)

This commit adds checks for the end of the string to pm_regexp_parse_range_quantifier.