Avoid assertion failure in error_message

r17942 added a call to get_thread_buffer in the first part of
error_message, prior to the call to com_err_initialize.  This can
cause an assertion failure from k5_getspecific if error_message is
called on a system error before any other com_err functions are
called.  Move the initialization call earlier to prevent this.

ticket: 7822 (new)
target_version: 1.12.1
tags: pullup
Allow empty store in gss_acquire_cred_from

There is no reason to deny a zero-length cred store, so don't check
for it in val_acq_cred_args or val_add_cred_args.
Add test for gss_acquire_cred_from rcache feature