perf ui gtk: Ensure not to call gtk_main_quit() twice

Currently the gtk_main_quit() is called twice when perf exits so the
following warning is emitted:

  [penberg@tux perf]$ ./perf report --gtk
  ^Cperf: Interrupt

  (perf:4048): Gtk-CRITICAL **: IA__gtk_main_quit: assertion `main_loops != NULL' failed

Fix it by not to call it unnecessarily.
Reported-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1345222583-3964-1-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/gtk/setup.c

@@ -13,6 +13,8 @@ int perf_gtk__init(void)
 
 void perf_gtk__exit(bool wait_for_ok __used)
 {
+	if (!perf_gtk__is_active_context(pgctx))
+		return;
 	perf_error__unregister(&perf_gtk_eops);
 	gtk_main_quit();
 }