rtsp-media: fix double unlock in _get_buffer_size()

Fixes an abort when calling gst_rtsp_media_get_buffer_size()
because of double g_mutex_unlock () usage.

https://bugzilla.gnome.org/show_bug.cgi?id=745434

gst/rtsp-server/rtsp-media.c

@@ -1083,7 +1083,7 @@ gst_rtsp_media_get_buffer_size (GstRTSPMedia * media)
 
   priv = media->priv;
 
-  g_mutex_unlock (&priv->lock);
+  g_mutex_lock (&priv->lock);
   res = priv->buffer_size;
   g_mutex_unlock (&priv->lock);