Set retval in the empty password case to avoid a path through the

code that fails to set retval before falling through to the final
return().

Reported by:	emaste
Reported by:	Coverity
CID:		1018711
MFC after:	1 week

lib/libpam/modules/pam_unix/pam_unix.c

@@ -332,6 +332,7 @@ pam_sm_chauthtok(pam_handle_t *pamh, int flags,
 			 * XXX check PAM_DISALLOW_NULL_AUTHTOK
 			 */
 			old_pass = "";
+			retval = PAM_SUCCESS;
 		} else {
 			retval = pam_get_authtok(pamh,
 			    PAM_OLDAUTHTOK, &old_pass, NULL);