gpollfilemonitor: don't emit after cancellation

GPollFileMonitor emits CHANGES_DONE_HINT after CHANGED signals, but it
doesn't check to ensure that the file monitor wasn't cancelled before it
does that.

If the original signal caused the monitor to be unreffed, cancelled and
destroyed, we would still end up emitting an extra signal on it.

Avoid that by checking first for cancellation.

https://bugzilla.gnome.org/show_bug.cgi?id=739424