Silence Coverity complaint about possible null-pointer dereference.

Silence Coverity complaint about possible null-pointer dereference.

If pg_init_privs were to contain a NULL ACL field, this code would
pass old_acl == NULL to merge_acl_with_grant, which would crash.
The case shouldn't happen, but it just takes a couple more lines
of code to guard against it, so do so.

Oversight in 534287403; no back-patch needed.