ext/taglib/gstid3v2mux.cc: Handle  writing of track-count or album-volume-count without track-number or albume-volume...

Original commit message from CVS:
* ext/taglib/gstid3v2mux.cc:
Handle  writing of track-count or album-volume-count without
track-number or albume-volume-number (in this case the number
will just be set to 0).
* tests/check/elements/id3v2mux.c: (test_taglib_id3mux_check_tags):
It would be nice if we actually checked the values received for
track/album-volume number/count in  _check_tags(), rather than
setting them again ...