Fix stdin ending up not registered after a Quit

Fix stdin ending up not registered after a Quit

If you press Ctrl-C while GDB is processing breakpoint commands the
TRY/CATCH in inferior_event_handler catches the Quit exception and
prints it, and then if the interpreter was running a foreground
execution command, nothing re-adds stdin back in the event loop,
meaning the debug session ends up busted, because the user can't type
anything...

This was exposed by the new gdb.base/bp-cmds-continue-ctrl-c.exp
testcase added later in the series.

gdb/ChangeLog:
2017-11-16  Pedro Alves  <palves@redhat.com>

	* inf-loop.c (inferior_event_handler): Don't swallow the exception
	if the prompt is blocked.