avformat/avidec: match first index and first packet size=0 handling

Fixes Ticket2861

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 227a0eb5a9)

Conflicts:
	libavformat/avidec.c

libavformat/avidec.c

@@ -1303,7 +1303,7 @@ static int avi_read_idx1(AVFormatContext *s, int size)
         st = s->streams[index];
         ast = st->priv_data;
 
-        if(first_packet && first_packet_pos && len) {
+        if (first_packet && first_packet_pos) {
             data_offset = first_packet_pos - pos;
             first_packet = 0;
         }