verify intptr_t round-trip through void *

Round-trip of intptr_t through void * isn't explicitly guaranteed by C99. There are implementation-defined behaviors for integer to void *, and vice versa. Only a round-trip of void * through intptr_t is guaranteed, not vice versa. We seem to depend on being able to store an integer in a void * from time to time, e.g., to store integers in a smartlist.

We should run some automated tests to make sure this works as we expect, at least until we stop needing this behavior. (See also #23714 (moved).)