avcodec/av1_parser: initialize avctx->pix_fmt

Initialize avctx->pix_fmt in av1_parser.c
AV1 Chroma format is invalid when quering using below code if no AV1 decoder
is available:

iVideoStream = av_find_best_stream(fmtc, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
eChromaFormat = (AVPixelFormat)fmtc->streams[iVideoStream]->codecpar->format;

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/av1_parser.c

@@ -171,6 +171,8 @@ static int av1_parser_parse(AVCodecParserContext *ctx,
         color->transfer_characteristics  == AVCOL_TRC_IEC61966_2_1)
         ctx->format = pix_fmts_rgb[color->high_bitdepth + color->twelve_bit];
 
+    avctx->pix_fmt = ctx->format;
+
     avctx->profile = seq->seq_profile;
     avctx->level   = seq->seq_level_idx[0];