lib/pwdutils: use assert to check correct usage.

lib/pwdutils: use assert to check correct usage.

Since these functions are only used internally, we can make sure they
are being used correctly, and assert() helps in catching remaining
issues. Usage of each changed function has been reviewed:

For xgetpwnam:

- chsh(1) only calls it if a username has been set
- login(1) only calls it if username has been set and is not empty
- su(1) always initializes new_user to "root"
- unshare(1) calls get_user with optarg, so always set as well

For xgetgrnam:

- unshare(1) calls get_group with optarg

For xgetpwuid:

- chsh(1) passes a stack allocated struct for struct passwd

Signed-off-by: Érico Nogueira <erico.erc@gmail.com>