setregid02: invalid user value fix

Commit 705926d096f7a64c9d53efeb789288e25457d68f
introduced a bug that the value for inval_user was treated
as '-1' when passed to 16-bit version of setregid().

Fixed it the same way as it's done for setreuid test cases.

Reported-by: Honggyu Kim <hong.gyu.kim@lge.com>
Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>