Make sure we don't pass UTF-8 to SDL_iscntrl()

This function is only valid for values <= 127.

Closes https://github.com/libsdl-org/SDL/pull/8637

src/events/SDL_keyboard.c

@@ -1063,7 +1063,7 @@ int SDL_SendKeyboardText(const char *text)
     int posted;
 
     /* Don't post text events for unprintable characters */
-    if (SDL_iscntrl((int)*text)) {
+    if (!(*text & 0x80) && SDL_iscntrl(*text)) {
         return 0;
     }