avformat/avidec: Workaround broken initial frame

Fixes Ticket4851
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -1568,7 +1568,8 @@ static int avi_read_idx1(AVFormatContext *s, int size)
         ast = st->priv_data;
 
         if (first_packet && first_packet_pos) {
-            data_offset  = first_packet_pos - pos;
+            if (avi->movi_list + 4 != pos || pos + 500 > first_packet_pos)
+                data_offset  = first_packet_pos - pos;
             first_packet = 0;
         }
         pos += data_offset;