[Bug c/96739] attribute(constructor) vs format NULL check

[dgilbert at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96739

--- Comment #2 from Dr. David Alan Gilbert <dgilbert at redhat dot com> ---
Created attachment 49108
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49108&action=edit
less boiled down test