New xserver dix/grabs.c code can cause crash in Xming

[Colin Harrison]

After the inclusion of the recent function GrabsAreIdentical() in
dix/grabs.c,
(fix from bug 2738)
I get an occasional server crash, on client start-up, at line 341..

pGrab->next = pGrab->window->optional->passiveGrabs;

due to pGrab->window->optional being 0

More trace can be supplied if required.

Thanks
Colin Harrison