gst/gsttagsetter.c: gst_tag_list_merge() returns a new list, so it's not the best...

gst/gsttagsetter.c: gst_tag_list_merge() returns a new list, so it's not the best idea to ingore its return value. Ef...

Original commit message from CVS:
Patch by: René Stadler <mail at renestadler dot de>
* gst/gsttagsetter.c: (gst_tag_setter_merge_tags):
gst_tag_list_merge() returns a new list, so it's not the best idea
to ingore its return value. Effectively meant that tags could only
be merged on a GstTagSetter once using _merge_tags(). Fixes #395554.
Also add function guard to require a non-NULL taglist as input (has
always been so due to gst_tag_list_copy(), just making it explicit).