lavc: initialize AVCodecContext.sw_pix_fmt properly

Currently it's memset to 0, which is YUV420P. It should be initialized
to none.

libavcodec/options.c

@@ -98,6 +98,7 @@ static int init_context_defaults(AVCodecContext *s, const AVCodec *codec)
     s->execute2            = avcodec_default_execute2;
     s->sample_aspect_ratio = (AVRational){0,1};
     s->pix_fmt             = AV_PIX_FMT_NONE;
+    s->sw_pix_fmt          = AV_PIX_FMT_NONE;
     s->sample_fmt          = AV_SAMPLE_FMT_NONE;
 
     s->reordered_opaque    = AV_NOPTS_VALUE;