randr: Bail from RRTellChanged if there's no root window yet

This can happen if RRTellChanged is called during initialization.

Continuing in that case makes no sense conceptually:

* Any event sent over the wire requires a corresponding window.
* No root window probably means there can't be any clients which could
  receive the events.

In practice, it would result in a crash down the road due to
dereferencing the NULL ScreenRec::root pointer.
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>

randr/randr.c

@@ -615,6 +615,10 @@ RRTellChanged(ScreenPtr pScreen)
         primarysp = pScrPriv;
     }
 
+    /* If there's no root window yet, can't send events */
+    if (!primary->root)
+        return;
+
     xorg_list_for_each_entry(iter, &primary->secondary_list, secondary_head) {
         pSecondaryScrPriv = rrGetScrPriv(iter);