mgetgroups: avoid argument promotion issues with -1

mgetgroups: avoid argument promotion issues with -1

On platforms where gid_t is equivalent to uint16_t, argument
promotion states that -1 != (gid_t) -1.

* lib/mgetgroups.c (mgetgroups): A cast is required when checking
for invalid gid_t.
* tests/test-chown.h (getegid, test_chown): Likewise.
* tests/test-lchown.h (getegid, test_lchown): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>