This will likely also fix CID 1452427, a false positive resulting from
Coverity thinking that av_dict_set() automatically frees its key and
value parameters (even without the AV_DICT_DONT_STRDUP_* flags).

(AV_DICT_APPEND and AV_DICT_DONT_STRDUP_VAL are compatible with each
other since a8c5b455, so we can reset this flag here. It has originally
been removed in 0dc66553 when appending was added.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>