audio: Don't free hw resources until after hw backend is stopped

When stopping an audio voice, call the audio backend's fini
method before calling audio_pcm_hw_free_resources_ rather than
afterwards. This allows backends which use helper threads (like
pulseaudio) to terminate those threads before the conv_buf or
mix_buf are freed and avoids race conditions where the helper
may access a NULL pointer or freed memory.

Cc: qemu-stable@nongnu.org
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1418406239-9838-1-git-send-email-peter.maydell@linaro.org
(cherry picked from commit b28fb27b5edf77f6fd0ac550a156fb20f2218db3)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

audio/audio_template.h

@@ -191,9 +191,9 @@ static void glue (audio_pcm_hw_gc_, TYPE) (HW **hwp)
         audio_detach_capture (hw);
 #endif
         QLIST_REMOVE (hw, entries);
+        glue (hw->pcm_ops->fini_, TYPE) (hw);
         glue (s->nb_hw_voices_, TYPE) += 1;
         glue (audio_pcm_hw_free_resources_ ,TYPE) (hw);
-        glue (hw->pcm_ops->fini_, TYPE) (hw);
         g_free (hw);
         *hwp = NULL;
     }