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

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.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>