Fixed assertion in mpack_reader_init for the case when data are NULL.

Signed-off-by: Marat Abrarov <abrarov@gmail.com>

Author: mabrarov

src/mpack/mpack-reader.c

@@ -30,7 +30,7 @@ MPACK_SILENCE_WARNINGS_BEGIN
 static void mpack_reader_skip_using_fill(mpack_reader_t* reader, size_t count);
 
 void mpack_reader_init(mpack_reader_t* reader, char* buffer, size_t size, size_t count) {
-    mpack_assert(buffer != NULL, "buffer is NULL");
+    mpack_assert(count == 0 || buffer != NULL, "buffer is NULL for for %u bytes", count);
 
     mpack_memset(reader, 0, sizeof(*reader));
     reader->buffer = buffer;