mpeg2dec: fix spurious failure to decode some files

Parsing can return with an 'invalid' state, but this is not
actually fatal. For one, the mpeg2dec command line tool that
comes with the libmpeg2 library blithely ignores this condition
and merrily goes on. So we do this same, logging the error,
and going on with parsing. This makes something work that did
not use to work, and brings happiness to the world.

https://bugzilla.gnome.org/show_bug.cgi?id=429476

ext/mpeg2dec/gstmpeg2dec.c

@@ -1150,7 +1150,7 @@ gst_mpeg2dec_chain (GstPad * pad, GstBuffer * buf)
               ("%d consecutive decoding errors", mpeg2dec->error_count),
               (NULL));
         }
-        goto exit;
+        continue;
       default:
         GST_ERROR_OBJECT (mpeg2dec, "Unknown libmpeg2 state %d, FIXME", state);
         goto exit;