Fix warning about signedness of pointers

[Mike McCormack]

Robert Shearman wrote:
> Vincent Béron wrote:
> 
>> Changelog:
>> Fix warning about signedness of pointers in x11drv.

> Mike McCormack already tried an identical patch, but it was rejected 
> because the implications of the fix were not considered. It doesn't seem 
> that they were addressed in this patch either. IIRC, the main issue was 
> that *p can in fact be negative.

That bit of code in event.c is a bit of a mess, and it probably needs 
rewriting and retesting rather than just fixing the warnings.

Mike