Fix 64-bit Windows socket write error handling

Add casts to ensure that the result type of SOCKET_WRITEV() on Windows
can represent -1.  Otherwise it will be treated as 2^32-1 when cast to
ssize_t on 64-bit Windows, which can lead to crashes in
krb5_sendto_kdc().  Reported by Puran Chand.

https://github.com/krb5/krb5/commit/d98c3238894986765631f1e556f29ca817988816
Author: Greg Hudson <ghudson@mit.edu>
Commit: d98c3238894986765631f1e556f29ca817988816
Branch: master
 src/include/port-sockets.h  |    5 +++--
 src/lib/krb5/os/net_write.c |    2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)