Initialize err variable in krb5_sendto_kdc

When we get an KRB5_KDC_UNREACH error back from k5_sendto, we check if
the err variable we passed for use by our message handler has been set
to KDC_ERR_SVC_UNAVAILABLE.  If k5_sendto doesn't receive any
response, though, the handler isn't called, so we're reading an
uninitialized variable.  Initialize it to a value other than
KDC_ERR_SVC_UNAVAILABLE to be sure.

[ghudson@mit.edu: initialize err just before calling k5_sendto; edit
commit message]

ticket: 7874 (new)
target_version: 1.12.2
tags: pullup

src/lib/krb5/os/sendto_kdc.c

@@ -214,6 +214,7 @@ krb5_sendto_kdc(krb5_context context, const krb5_data *message,
     if (retval)
         return retval;
 
+    err = 0;
     retval = k5_sendto(context, message, &servers, socktype1, socktype2,
                        NULL, reply, NULL, NULL, &server_used,
                        check_for_svc_unavailable, &err);