asfdec_o: prevent overflow causing seekback

This fixes infinite loops.

Reviewed-by: Alexandra Hájková <alexandra.khirnova@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/asfdec_o.c

@@ -167,7 +167,7 @@ static void swap_guid(ff_asf_guid guid)
 
 static void align_position(AVIOContext *pb,  int64_t offset, uint64_t size)
 {
-    if (avio_tell(pb) != offset + size)
+    if (size < INT64_MAX - offset && avio_tell(pb) != offset + size)
         avio_seek(pb, offset + size, SEEK_SET);
 }