logind: use memdup_suffix0() where appropiate

memdup_suffix0() is the call of choice when we are dealing with an input
that might not be NUL terminated. strndup() assumes NUL termination.

src/login/logind-utmp.c

@@ -43,7 +43,7 @@ int manager_read_utmp(Manager *m) {
                 if (!pid_is_valid(u->ut_pid))
                         continue;
 
-                t = strndup(u->ut_line, sizeof(u->ut_line));
+                t = memdup_suffix0(u->ut_line, sizeof(u->ut_line));
                 if (!t)
                         return log_oom();