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