vp9parser: initialize subsampling to -1

The default value of 0 is a valid subsampling value and could be
confused with an 'unset' value.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3260>

subprojects/gst-plugins-bad/gst-libs/gst/codecparsers/gstvp9parser.c

@@ -646,6 +646,7 @@ gst_vp9_parser_new (void)
     return NULL;
 
   parser->priv = priv;
+  parser->subsampling_x = parser->subsampling_y = -1;
 
   return parser;
 }