tweaks: avoid running tolower() on an out-of-range value

Functions like tolower() expect characters in the range -1..255.

Reference: https://savannah.gnu.org/bugs/?50289.

The other occurrences of toupper(), tolower(), and isxdigit() that
don't do a cast are fine, because the values of 'code' or 'keycode'
are already guaranteed to be in range by the surrounding code.

src/global.c

@@ -507,7 +507,7 @@ functionptrtype func_from_key(const int keycode)
  * with Pico or to mimic 'less' and similar text viewers. */
 functionptrtype interpret(const int keycode)
 {
-	if (!meta_key) {
+	if (!meta_key && keycode < 0x7F) {
 		if (keycode == 'N')
 			return do_findprevious;
 		if (keycode == 'n')