dix/privates.c: Avoid undefined behaviour after realloc()

Adding the offset between the realloc result and the old allocation to update pointers into the new allocation is undefined behaviour: the old pointers are no longer valid after realloc() according to the C standard. While this works on almost all architectures and compilers, it causes problems on architectures that track pointer bounds (e.g. CHERI or Arm's Morello): the DevPrivateKey pointers will still have the bounds of the previous allocation and therefore any dereference will result in a run-time trap.

I found this due to a crash (dereferencing an invalid capability) while trying to run XVnc on a CHERI-RISC-V system. With this commit I can successfully connect to the XVnc instance running inside a QEMU with a VNC viewer on my host.

This also changes the check whether the allocation was moved to use uintptr_t instead of a pointer since according to the C standard: "The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime." Casting to an integer type avoids this undefined behaviour.