login: use mem2strcpy() rather than rely on printf()

login: use mem2strcpy() rather than rely on printf()

The strings from utmp does not have to be terminated. It's seems
better to explicitly terminate it than rely on "%.*s" printf()
functionality -- printf() man page assumes that "If a precision is
given, no null byte need be present", but static analyzers are pretty
unhappy with it.

Signed-off-by: Karel Zak <kzak@redhat.com>