tests: fix the tag test
tests: fix the tag test

The previous change (see bgo #756069) was causing us to free the same pointer
multiple times. If we actually get a sample back, the test fails, no need to
free anything in that case.

StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer