patch 9.1.1395: search_stat not reset when pattern differs in case

patch 9.1.1395: search_stat not reset when pattern differs in case

Problem:  search_stat not reset when pattern differs in case
          (tahzibijafar)
Solution: use STRNCMP instead of MB_STRNICMP macro

There was a long standing todo comment, that using MB_STRNICMP is wrong.
So let's change it to STRNCMP() instead. Even if it not handle
multi-byte characters correctly, then Vim will rather recompute the
search stat, instead of re-using the old (and possibly wrong) value.

fixes: #17312
closes: #17314

Signed-off-by: Christian Brabandt <cb@256bit.org>