kdb: Permit a NULL thread credential in kdb_backend_permitted()

Early during boot, thread0 runs with td->td_ucred == NULL. This is
fixed up in proc0_init() at SI_SUB_INTRINSIC. If a panic occurs before
then, rather than dereference a NULL pointer, simply allow the thread to
enter KDB.

Reported by: stevek
Reviewed by: mhorne, stevek
MFC after: 1 week
Fixes: cab1056105e3 ("kdb: Modify securelevel policy")
Differential Revision: https://reviews.freebsd.org/D41280