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; }