properly bail out when PAM changes username

OpenSSH doesn't support PAM changing its conception of the
username via a module calling pam_set_item(h, PAM_USER, ...).
We were supposed to bail out here, but I messed up while "fixing"
this last time and dropped a return statement.

Reported by Mike Damm

auth-pam.c

@@ -474,6 +474,7 @@ check_pam_user(Authctxt *authctxt)
 	if (strcmp(sshpam_initial_user, pam_user) != 0) {
 		error_f("PAM user \"%s\" does not match previous \"%s\"",
 		      pam_user, sshpam_initial_user);
+		return PAM_USER_UNKNOWN;
 	}
 	return PAM_SUCCESS;
 }