posix: shm: ensure addend is less than INTPTR_MAX

Although it's quite unlikely that we will see shared memory
objects in practice that are greater than 2GiB, there is a
possibility of integer overflow when comparing intptr_t and
size_t, since the former is signed and the latter is unsigned.

Explicitly check to ensure that the addend is less than
INTPTR_MAX before subtraction.

This fixes CID 487734

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>