libkrb5: Fix potential double-free

If krb5_make_principal fails, tmp_creds.server may remain a pointer to freed
memory and then be double-freed.  After freeing it the first time, initialize
it to NULL, which causes subsequent krb5_free_principal calls to do the right
thing.

Reported by:	Coverity
CID:		1273430
Sponsored by:	EMC / Isilon Storage Division

crypto/heimdal/lib/krb5/get_cred.c

@@ -831,6 +831,7 @@ get_cred_kdc_capath_worker(krb5_context context,
 	if(strcmp(tgt_inst, server_realm) == 0)
 	    break;
 	krb5_free_principal(context, tmp_creds.server);
+	tmp_creds.server = NULL;
 	ret = krb5_make_principal(context, &tmp_creds.server,
 				  tgt_inst, KRB5_TGS_NAME, server_realm, NULL);
 	if(ret) {