Change list-length intptr_t to ptrdiff_t

Change list-length intptr_t to ptrdiff_t

* src/fns.c (list_length, Fsafe_length, Fproper_list_p):
Use ptrdiff_t not intptr_t for accumulator, since result is ptrdiff_t.
This fixes a minor glitch in 2019-01-11T05:35:31!eggert@cs.ucla.edu
where I removed unnecessary overflow checks but forgot to change types.
This change should alter generated code only on oddball platforms where
ptrdiff_t is narrower than intptr_t, e.g., CheriBSD.

(cherry picked from commit 76497a01425e19a6c3a02c1e3031061fa0e7885b)