matroskademux: Unlock stream lock after use

Stream lock of sink pad was not unlocked after non-updating seek.

gst/matroska/matroska-demux.c

@@ -2092,6 +2092,7 @@ next:
   /* ouch */
   if (!update) {
     GST_PAD_STREAM_LOCK (demux->common.sinkpad);
+    pad_locked = TRUE;
     goto exit;
   }