cogl-context: Initialize context->stub_winsys

If Cogl is built using a full Winsys then it wasn't initialising
context->stub_winsys which sometimes made it crash later on.

clutter/cogl/cogl/cogl-context.c

@@ -154,6 +154,7 @@ cogl_context_new (CoglDisplay *display,
   context->display = display;
 
 #ifdef COGL_HAS_FULL_WINSYS
+  context->stub_winsys = FALSE;
   if (!_cogl_winsys_context_init (context, error))
     {
       cogl_object_unref (display);