qtmux: Fix sample memory leak

When getting sample from taglist, the memory should be freed after
using it.

https://bugzilla.gnome.org/show_bug.cgi?id=756068

gst/isomp4/gstqtmux.c

@@ -1414,6 +1414,7 @@ gst_qt_mux_add_metadata_tags (GstQTMux * qtmux, const GstTagList * list,
         }
         gst_buffer_unmap (buf, &map);
       }
+      gst_sample_unref (sample);
     }
   }