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.)