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>

ffmpeg.c

@@ -527,6 +527,8 @@ static void ffmpeg_cleanup(int ret)
         av_freep(&ost->audio_channels_map);
         ost->audio_channels_mapped = 0;
 
+        av_dict_free(&ost->sws_dict);
+
         avcodec_free_context(&ost->enc_ctx);
 
         av_freep(&output_streams[i]);