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>
diff --git a/src/search.c b/src/search.c
index ea7e654..14990e2 100644
--- a/src/search.c
+++ b/src/search.c
@@ -3291,12 +3291,9 @@
 	       || (dirc == '/' && LT_POS(p, lastpos)));
 
     // If anything relevant changed the count has to be recomputed.
-    // MB_STRNICMP ignores case, but we should not ignore case.
-    // Unfortunately, there is no MB_STRNICMP function.
-    // XXX: above comment should be "no MB_STRCMP function" ?
     if (!(chgtick == CHANGEDTICK(curbuf)
 	&& (lastpat != NULL
-	    && MB_STRNICMP(lastpat, spats[last_idx].pat, lastpatlen) == 0
+	    && STRNCMP(lastpat, spats[last_idx].pat, lastpatlen) == 0
 	    && lastpatlen == spats[last_idx].patlen
 	)
 	&& EQUAL_POS(lastpos, *cursor_pos)