asfdec: Fix reading from the pipe

Signed-off-by: NLuca Barbato <lu_zero@gentoo.org>

libavformat/asfdec.c

@@ -922,7 +922,8 @@ static int asf_read_data(AVFormatContext *s, const GUIDParseTable *g)
                size, asf->nb_packets);
     avio_skip(pb, 2); // skip reserved field
     asf->first_packet_offset = avio_tell(pb);
-    align_position(pb, asf->offset, asf->data_size);
+    if (pb->seekable)
+        align_position(pb, asf->offset, asf->data_size);
 
     return 0;
 }