This problem is very difficult to reproduce. It did
happen all the time with the command:

$ su -

  The analysis of the code path shows:

    if (!(announce & LASTLOG_QUIET)) {

	if (last_login.ll_time) {

	    /* we want the date? */
	    if (announce & LASTLOG_DATE) {
	        struct tm *tm, tm_buf;
		time_t ll_time;

		ll_time = last_login.ll_time;
		tm = localtime_r (&ll_time, &tm_buf);
		strftime (the_time, sizeof (the_time),
	        /* TRANSLATORS: "strftime options for date of last login" */
			  _(" %a %b %e %H:%M:%S %Z %Y"), tm);

  The localtime_r call returns NULL, then it pass NULL
to strftime, and crashes.

  It should check the return value of localtime_r before
calling strftime.

  The user was told to execute:

# sed -e 's/\(pam_lastlog.so.*\)/\1 nodate/g' -i /etc/pam.d/postlogin

that fixed the problem, but more interestingly, after
reverting the sed script change, it continued to work,
so, the problem appears to have been a corrupted
last_login.ll_time, that may have been a corrupted
/var/log/lastlog