playsink: Fix volume element leak

In case sink implements a streamvolume interface, volume element is being got
from the sink. But this is transfer full. So the memory should be freed before
setting it to NULL. This was resulting in major memory leaks

https://bugzilla.gnome.org/show_bug.cgi?id=755867

gst/playback/gstplaysink.c

@@ -3472,6 +3472,8 @@ gst_play_sink_do_reconfigure (GstPlaySink * playsink)
 
         activate_chain (GST_PLAY_CHAIN (playsink->audiochain), FALSE);
         disconnect_audio_chain (playsink->audiochain, playsink);
+        if (playsink->audiochain->volume)
+          gst_object_unref (playsink->audiochain->volume);
         playsink->audiochain->volume = NULL;
         if (playsink->audiochain->ts_offset)
           gst_object_unref (playsink->audiochain->ts_offset);
@@ -3580,6 +3582,8 @@ gst_play_sink_do_reconfigure (GstPlaySink * playsink)
 
       if (playsink->audiochain->sink_volume) {
         disconnect_audio_chain (playsink->audiochain, playsink);
+        if (playsink->audiochain->volume)
+          gst_object_unref (playsink->audiochain->volume);
         playsink->audiochain->volume = NULL;
         if (playsink->audiochain->ts_offset)
           gst_object_unref (playsink->audiochain->ts_offset);
@@ -4786,6 +4790,8 @@ gst_play_sink_change_state (GstElement * element, GstStateChange transition)
         /* remove our links to the volume elements when they were
          * provided by a sink */
         disconnect_audio_chain (playsink->audiochain, playsink);
+        if (playsink->audiochain->volume)
+          gst_object_unref (playsink->audiochain->volume);
         playsink->audiochain->volume = NULL;
       }