Created attachment 303986 [details] [review]
patch which reduces race probability

I see the following behavior in thunderbird (on Save As dialog). Sometimes the same crash can occur in nautilus. Unfortunately I couldn't reproduce the crash in some synthetic example. 

1) file monitor is being disposed,   
2) emit_in_idle is called
3) signal is registered and emit_cb is called
4) when emit_cb is called, monitor is already disposed and application crashes on the first line of emit_cb (GFileMonitor *monitor = G_FILE_MONITOR (data) ). 

It seems that checking for priv->cancelled in emit_in_idle fixes this particular issue at least in thunderbird.  But I'm not sure that it's enough as when g_file_monitor_dispose() and emit_in_idle() are called in parallel
g_file_monitor_dispose() can clean priv->pending_file_change_source and not call g_file_monitor_cancel() yet. In this case emit_in_idle() can not detect that monitor is being disposed and still register emit_cb() callback.