vc1_parser: fix parsing of the frame headers in interlaced streams

first_pic_header_flag needs to be set to allow the parsing code to change
some stream parameters, and not error out.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1_parser.c

@@ -45,6 +45,7 @@ static void vc1_extract_headers(AVCodecParserContext *s, AVCodecContext *avctx,
 
     vpc->v.s.avctx = avctx;
     vpc->v.parse_only = 1;
+    vpc->v.first_pic_header_flag = 1;
     next = buf;
     s->repeat_pict = 0;