Convert success in krb5_chpw_result_code_string

Result code 0 used to be converted properly by krb5_set_password,
though not krb5_change_password; this changed in 1.10 when
krb5int_setpw_result_code_string was folded into
krb5_chpw_result_code_string.  Restore the old behavior, and make it
apply to krb5_change_password as well, by making
krb5_chpw_result_code_string convert result code 0.

[ghudson@mit.edu: commit message]

(cherry picked from commit 316bf74)

ticket: 7569
version_fixed: 1.11.1
status: resolved

src/lib/krb5/krb/chpw.c

@@ -269,6 +269,9 @@ krb5_chpw_result_code_string(krb5_context context, int result_code,
     case KRB5_KPASSWD_INITIAL_FLAG_NEEDED:
         *code_string = _("Initial password required");
         break;
+    case 0:
+        *code_string = _("Success");
+        break;
     default:
         *code_string = _("Password change failed");
         break;