xvimagesink: fix leak when images are freed after the X context

I'm not 100% sure this is valid on any other X server than mine,
but since the XFree call does not take the context as a parameter,
it seems pretty certain it's the right thing to do, but I'll put
this caveat here in case someone checks in the future.

sys/xvimage/xvimagesink.c

@@ -244,6 +244,8 @@ gst_xvimage_buffer_destroy (GstXvImageBuffer * xvimage)
       shmdt (xvimage->SHMInfo.shmaddr);
     }
 #endif
+    if (xvimage->xvimage)
+      XFree (xvimage->xvimage);
     goto beach;
   }