winpthreads: mutex.c: update static function `mutex_impl`

This function now returns zero on success and an error code on failure.
This allows to propagate error code from `pthread_mutex_init` to the caller.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>