basevideoencoder: fix element leak

and this concludes an hour of yelling at the bloody test failing,
only to track down the problem not being in the test.

https://bugzilla.gnome.org/show_bug.cgi?id=657368

gst-libs/gst/video/gstbasevideoencoder.c

@@ -432,6 +432,7 @@ done:
 
   GST_LOG_OBJECT (base_video_encoder, "Returning caps %" GST_PTR_FORMAT, fcaps);
 
+  g_object_unref (base_video_encoder);
   return fcaps;
 }