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

[Peter Hutterer]

Colin Harrison wrote:
> 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.

my fault. deleting a passive grab may delete the window optional, if the 
  window in question is not the root window. my test app was too simple 
to spot that.

anyway, fixed and pushed. thanks for that.

Cheers,
    Peter