Do not call krb5_get_error_message with NULL context
MIT's krb5_get_error_message() ignores the context and can
be called with a NULL context. Heimdal's version does not.
Derived from http://gerrit.openafs.org/5508 aka commit
9dd9cfa0e1536e0e75628c84605b3d5b8486d69c
The 1.6 branch is sufficiently different from master.
Change-Id: Ie325705a78b155a04a6a9d44800037b72f5b045c
Reviewed-on: http://gerrit.openafs.org/5569
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>