fix MITKRB5-SA-2006-002 for 1.5-branch

fix MITKRB5-SA-2006-002 for 1.5-branch

pull up r19042 from trunk

 r19042@cathode-dark-space:  tlyu | 2007-01-09 14:45:10 -0500
 ticket: new
 target_version: 1.6
 tags: pullup
 subject: MITKRB5-SA-2006-002: svctcp_destroy() can call uninitialized function pointer
 component: krb5-libs

 Explicitly null out xprt->xp_auth when AUTH_GSSAPI is being used, so
 that svctcp_destroy() will not call through an uninitialized function
 pointer after code in svc_auth_gssapi.c has destroyed expired state
 structures.  We can't unconditionally null it because the RPCSEC_GSS
 implementation needs it to retrieve state.

ticket: new
tags: pullup
target_version: 1.5.2
version_fixed: 1.5.2
component: krb5-libs

git-svn-id: svn://anonsvn.mit.edu/krb5/branches/krb5-1-5@19049 dc483132-0cff-0310-8789-dd5450dbe970