Make aw-make-frame disable-able

There are times when I don't want a new frame made under any
circumstances. If that's the case, I don't want to accidentally
trigger aw-make-frame by hitting the wrong key at the dispatch.
Setting aw-make-frame-char to nil takes it off the key list, but this
requires a slight change to the dispatch check.

ace-window.el

@@ -431,7 +431,7 @@ The new frame is set to the same size as the previous frame, offset by
               (avy-mouse-event-window char)))
         ((= char (aref (kbd "C-g") 0))
          (throw 'done 'exit))
-        ((= char aw-make-frame-char)
+        ((and aw-make-frame-char (= char aw-make-frame-char))
          ;; Make a new frame and perform any action on its window.
          (let ((start-win (selected-window))
                (end-win (frame-selected-window (aw-make-frame))))