lib/utmp.c: Fixed generated strings for "ut_id"

lib/utmp.c: Fixed generated strings for "ut_id"

When no "ut_id" is provided by previous utmp file entry, "login" must
generate its own "ut_id". Historically tty number  was used for it.
However, if current device name is three characters or shorter, then
empty "ut_id" is produced or even garbage is copied from uninitialised
part of the "line".
This commit fixes it.
This patch uses unportable C extension Elvis operator as it is already
used everywhere in the code.

Signed-off-by: Evgeny Grin (Karlson2k) <k2k@drgrin.dev>