The documentation of av_dict_set() states that values >= 0 indicate
success, whereas av_packet_unpack_dictionary() implies that return
values > 0 are impossible. So only forward the return value of
av_dict_set() in av_packet_unpack_dictionary() on error.

(Btw: av_dict_set() does currently not return values > 0.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>