wpe: initialize threading.ready before reading it

Fix Valgrind warning.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7377>

subprojects/gst-plugins-bad/ext/wpe/gstwpethreadedview.cpp

@@ -73,6 +73,7 @@ GstWPEContextThread::GstWPEContextThread()
 {
     g_mutex_init(&threading.mutex);
     g_cond_init(&threading.cond);
+    threading.ready = FALSE;
 
     {
         GMutexHolder lock(threading.mutex);