avformat/dfa: Fix packet leak on error

Fixes: 07fa81f300736485e585b63e7b1cd155/signal_sigabrt_7ffff6ae7cb7_16_94ee815791a1357826524072db5bc2dc.dfa

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
