base-files: fix getting gid from group_add_next

base-files: fix getting gid from group_add_next

Shell function return code only has range [0, 255].  Other values will
be truncated, e.g. return 65536 will have the same effect as return 0

While at it, drop other "return $rc" where rc will almost always take
value 0 and whose value current callers actually do not check

Fixes FS#988

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>