Fix tq.el edge case

Fix tq.el edge case

(tq-queue-pop): Stifle error when a process has died and we are trying to
send a signal to it.  The program using tq.el should periodically check
to see whether the process has died and react appropriately -- this is
not the responsibility of tq.el, and is consistent with the rest of the
tq.el source code.

Revision: emacs@sv.gnu.org/emacs--devo--0--patch-779
Creator:  Michael Olson <mwolson@gnu.org>