bug#19494: 25.0.50; infinite loop in readable_event on master branch

[Noah Friedman <friedman@splode.com>]

Ever since commit 614beeecf29d16c08f66a4f82b6085be90df8a74 (c. Nov 01 2014)
I keep finding "idle" emacs processes running in a tight loop in
keyboard.c:readable_event after I come back to my session the following
morning.  In those cases, kbd_buffer is full of the same event, in my case:

        $17 = {
          kind = BUFFER_SWITCH_EVENT,
          part = scroll_bar_nowhere,
          code = 0,
          modifiers = 0,
          x = 0,
          y = 0,
          timestamp = 0,
          frame_or_window = 12962290,
          arg = 12962290
        }

That is, the entire kbd_buffer queue is BUFFER_SWITCH_EVENT events.
Because of that, it means the sequence

          do {
           ....
              if (event == kbd_buffer + KBD_BUFFER_SIZE)
                event = kbd_buffer;
	    }
	  while (event != kbd_store_ptr);

in the function readable_event will, at least at times, loop forever.

I don't have a trivial or always-reliable test case but I think the
offending elisp is the following, running in a (non-idle) timer:

         (force-mode-line-update 'all)
         (sit-for 0) ; force redisplay

I can change the elisp in question since I wrote it, but I'm not sure what
the right way to do it is since it's not just running during idle times.
In any case having emacs lock up in a non-recoverable way seems like a bug,
since the only way out is to hit C-g and abort, or else go in with gdb
on the running process and force a return.

Maybe this loop needs some kind of "didn't find any other event" flag to
avoid looping, but I thought I'd check with the original committer first in
case he has a better idea.