avidec: use actually read size instead of requested size

Fixes CVE-2012-2788

libavformat/avidec.c

@@ -1116,7 +1116,7 @@ resync:
             }
             ast->frame_offset += get_duration(ast, pkt->size);
         }
-        ast->remaining -= size;
+        ast->remaining -= err;
         if(!ast->remaining){
             avi->stream_index= -1;
             ast->packet_size= 0;