dashdemux: When clearing the sidx parser, reset all state

And especially don't keep entry count and index around, we have no
entries anymore after clearing.

ext/dash/gstisoff.c

@@ -363,7 +363,9 @@ void
 gst_isoff_sidx_parser_clear (GstSidxParser * parser)
 {
   g_free (parser->sidx.entries);
-  parser->sidx.entries = NULL;
+  memset (parser, 0, sizeof (*parser));
+
+  gst_isoff_sidx_parser_init (parser);
 }
 
 static void