Problem description:
smbd dump core in libkrb5-samba4.so.26.0.0:krb5_storage_free if arg. sp is NULL

Issue hits Samba using  Heimdal krb5 code only.

Promoted into upstream as Samba bug:
https://bugzilla.samba.org/show_bug.cgi?id=12505

--- a/source4/heimdal/lib/krb5/store.c	2017-01-09 16:03:14.204115997 -0800
+++ b/source4/heimdal/lib/krb5/store.c	2017-01-09 16:06:04.835337808 -0800
@@ -270,6 +270,7 @@
 KRB5_LIB_FUNCTION krb5_error_code KRB5_LIB_CALL
 krb5_storage_free(krb5_storage *sp)
 {
+    if(sp == NULL) return -1;
     if(sp->free)
 	(*sp->free)(sp);
     free(sp->data);