ffmpeg: fix sws_dict leak on error exit

Fixes: 1b79b985cdf860ffa228c00ee5497051/signal_sigsegv_1f99d24_3549_86d92054a79f6ff900fbaf03f8012b32.aif

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