Make sure to initialize 'err' 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.