Only clear crtc of output if it is the one we're actually working on.

Upon recreation of the RandR internal data structures in RRCrtcNotify() the
crtc of an output could be NULLed if the crtc was shared (cloned) between two
outputs and one of them got another crtc assigned.

randr/rrcrtc.c

@@ -150,7 +150,8 @@ RRCrtcNotify (RRCrtcPtr	    crtc,
 		break;
 	if (i == numOutputs)
 	{
-	    crtc->outputs[j]->crtc = NULL;
+	    if (crtc->outputs[j]->crtc == crtc)
+		crtc->outputs[j]->crtc = NULL;
 	    RROutputChanged (crtc->outputs[j], FALSE);
 	    RRCrtcChanged (crtc, FALSE);
 	}