kconfig: nconf: stop endless search loops

If the user selects the very first entry in a page and performs a
search-up operation, or selects the very last entry in a page and
performs a search-down operation that will not succeed (e.g., via
[/]asdfzzz[Up Arrow]), nconf will never terminate searching the page.

The reason is that in this case, the starting point will be set to -1
or n, which is then translated into (n - 1) (i.e., the last entry of
the page) or 0 (i.e., the first entry of the page) and finally the
search begins. This continues to work fine until the index reaches 0 or
(n - 1), at which point it will be decremented to -1 or incremented to
n, but not checked against the starting point right away. Instead, it's
wrapped around to the bottom or top again, after which the starting
point check occurs... and naturally fails.

My original implementation added another check for -1 before wrapping
the running index variable around, but Masahiro Yamada pointed out that
the actual issue is that the comparison point (starting point) exceeds
bounds (i.e., the [0,n-1] interval) in the first place and that,
instead, the starting point should be fixed.

This has the welcome side-effect of also fixing the case where the
starting point was n while searching down, which also lead to an
infinite loop.

OTOH, this code is now essentially all his work.

Amazingly, nobody seems to have been hit by this for 11 years - or at
the very least nobody bothered to debug and fix this.
Signed-off-by: Mihai Moldovan <ionic@ionic.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/nconf.c

@@ -508,8 +508,8 @@ static int get_mext_match(const char *match_str, match_f flag)
 	else if (flag == FIND_NEXT_MATCH_UP)
 		--match_start;
 
+	match_start = (match_start + items_num) % items_num;
 	index = match_start;
-	index = (index + items_num) % items_num;
 	while (true) {
 		char *str = k_menu_items[index].str;
 		if (strcasestr(str, match_str) != NULL)