Round-tripping pointers via gsize is not guaranteed to work (the C standard
only requires this for (u)inptr_t) and in fact breaks on CHERI-enabled
systems such as Arm Morello where pointers are 128-bits but size_t is 64.
This means the current casts would strip the high bits of the pointer and
return a non-dereferenceable value. Fix this by casting the operand that
holds the pointer to guintptr instead of gsize.
Helps: https://gitlab.gnome.org/GNOME/glib/-/issues/2842
ab7e584e9f