avformat/id3v2: Fix double-free on error

avformat/id3v2: Fix double-free on error

ff_id3v2_parse_priv_dict() uses av_dict_set() with the flags
AV_DICT_DONT_STRDUP_KEY and AV_DICT_DONT_STRDUP_VAL. In this case both
key and value are freed on error (and owned by the destination
dictionary on success), so that freeing them again on error is a
double-free and therefore forbidden. But it nevertheless happened.

Fixes CID 1452489 and 1452421.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>