tsdemux: reset iterator to NULL after it's free

Fixes possible invalid memory access in gst_ts_demux_stream_flush
diff --git a/gst/mpegtsdemux/tsdemux.c b/gst/mpegtsdemux/tsdemux.c
index 8b97b8e..8f4d78d 100644
--- a/gst/mpegtsdemux/tsdemux.c
+++ b/gst/mpegtsdemux/tsdemux.c
@@ -1547,6 +1547,7 @@
   memset (stream->pendingbuffers, 0, TS_MAX_PENDING_BUFFERS);
   stream->nbpending = 0;
   stream->current = NULL;
+  stream->currentit = NULL;
   stream->expected_size = 0;
   stream->current_size = 0;