chars: remove superfluous afterchecks

Now that mbstrncasecmp() does the right thing, there is no need any
more to verify that only a valid multibyte sequence was matched.

(See https://savannah.gnu.org/bugs/?45579 for a test case.)

Also, this will make it possible to search for invalid sequences.

(Currently it isn't possible to enter a search string with invalid
characters, but... a user might edit the search history file.  And
if pasting at the prompt is implemented, it will be trivial to enter
invalid sequences if you have a file that contains them.)

src/chars.c

@@ -594,8 +594,7 @@ char *mbstrcasestr(const char *haystack, const char *needle)
 
 	for (; *haystack != '\0' && haystack_len >= needle_len;
 		haystack += move_mbright(haystack, 0), haystack_len--) {
-	    if (mbstrncasecmp(haystack, needle, needle_len) == 0 &&
-			mblen(haystack, MB_CUR_MAX) > 0)
+	    if (mbstrncasecmp(haystack, needle, needle_len) == 0)
 		return (char *)haystack;
 	}
 
@@ -687,8 +686,7 @@ char *mbrevstrcasestr(const char *haystack, const char *needle, const
 
 	while (TRUE) {
 	    if (rev_start_len >= needle_len &&
-			mbstrncasecmp(rev_start, needle, needle_len) == 0 &&
-			mblen(rev_start, MB_CUR_MAX) > 0)
+			mbstrncasecmp(rev_start, needle, needle_len) == 0)
 		return (char *)rev_start;
 
 	    /* If we've reached the head of the haystack, we found nothing. */