been converted at all, set *last_wc to WEOF. If mbrtowc failed, set wc
to the byte which couldn't be converted.
(re_string_reconstruct): Don't clear valid_raw_len before calling
re_string_skip_chars. If wc is WEOF after re_string_skip_chars, set
tip_context using re_string_context_at.
* posix/Makefile: Add rules to build and run bug-regex25 test.
* posix/bug-regex25.c: New test.