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

[Noah Friedman]

>Why do you need that code, and in a timer on top of that?  It looks
>strangely, and probably papers over some bug, either one that existed
>in the past or maybe even an existing one.  If that bug still exists,
>a bug report will be appreciated.

It's very old code, circa 1995.  Since it predates the v21 redisplay engine
rewrite, I have no idea if it's still necessary; I'm testing that now.

>One thing is clear: having input-pending-p return non-nil when there's
>nothing in the queue but these buffer-switch events is a real bug that
>causes quite a few problems.  So going back is not an option, I
>think.

I concur.  I don't want to revert it, it may just need some refinement.