fftools/ffmpeg_dec: free decoder ctx after EOF

The codec context is no longer used after the decoder thread exits, but
still idly sticks around until program exit, wasting memory.

fftools/ffmpeg_dec.c

@@ -1018,6 +1018,7 @@ static int decoder_thread(void *arg)
 
 finish:
     dec_thread_uninit(&dt);
+    avcodec_free_context(&dp->dec_ctx);
 
     return ret;
 }