pull up r19195 from trunk

pull up r19195 from trunk

 r19195@cathode-dark-space:  jaltman | 2007-02-28 20:49:11 -0500
 ticket: new
 subject: krb5_get_cred_from_kdc fails to null terminate the tgt list
 tags: pullup

 	if the next tgt in a cross-realm traversal cannot be
 	obtained find_nxt_kdc() was calling krb5_free_creds()
 	on the last tgt in the list but was failing to nullify
 	the pointer to the cred that was just freed.

 	if there were no additional tgts obtained,
 	krb5_get_cred_from_kdc() would return a non-NULL terminated
 	cred list to the caller.  This would result in a crash
 	when attempting to manipulate the non-existent cred past
 	the end of the list.

 	This commit nullifies the credential pointer in
 	find_nxt_kdc() after the call to krb5_free_creds()

ticket: 5454
version_fixed: 1.6.1

git-svn-id: svn://anonsvn.mit.edu/krb5/branches/krb5-1-6@19197 dc483132-0cff-0310-8789-dd5450dbe970