Fix null dereference qualifying short hostnames

Fix the dnsglue.c PRIMARY_DOMAIN macro not to call strdup() with a
null pointer if no DNS search path is configured.

(cherry picked from commit cd82bf377e7fad2409c76bf8b241920692f34fda)

https://github.com/krb5/krb5/commit/c57b3f7f36c417c72ac66b014e705bf457babd26
Author: Greg Hudson <ghudson@mit.edu>
Commit: c57b3f7f36c417c72ac66b014e705bf457babd26
Branch: krb5-1.18
 src/lib/krb5/os/dnsglue.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)