The attached patch fixes the crash and Xorg appears to start normally.
--- xorg-server-1.18.0/randr/randr.c 2015-11-06 08:07:41.000000000 -0600
+++ xorg-server-1.18.0/randr/randr.c 2016-02-15 12:49:06.469645421 -0600
@@ -559,6 +559,9 @@
mastersp = pScrPriv;
}
+ if (!mastersp)
+ return;
+
mastersp->changed = TRUE;
}