Fix a use of uninitialized bytes in pecoff.c

This updates the local declaration of `str_size` to always be 4 bytes instead of
platform-dependent as its initialization later on only fills in 4 bytes instead
of all the bytes of `size_t`.

Originally reported as rust-lang/rust#28447 this was fixed in
rust-lang/rust#30908

pecoff.c

@@ -595,7 +595,7 @@ coff_add (struct backtrace_state *state, int descriptor,
   const b_coff_section_header *sects;
   struct backtrace_view str_view;
   int str_view_valid;
-  size_t str_size;
+  uint32_t str_size;
   off_t str_off;
   struct backtrace_view syms_view = {0};
   off_t syms_off;