Check for negative poll timeout in k5_sendto_kdc

(cherry picked from commit 74bee54227deb64a41c2e79f57dd2a2c5ea010a3)

https://github.com/krb5/krb5/commit/4fcd8cea7ee1594c25b9e7eec42ae3ce6775d93a
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 4fcd8cea7ee1594c25b9e7eec42ae3ce6775d93a
Branch: krb5-1.10
 src/lib/krb5/os/sendto_kdc.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)