Add null invalid pointer comment

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: carbotaniuman

src/intptrcast.rs

@@ -109,6 +109,7 @@ impl<'mir, 'tcx> GlobalStateInner {
         trace!("Casting 0x{:x} to a pointer", addr);
         let global_state = ecx.machine.intptrcast.borrow();
 
+        // Special-case NULL, it is always an invalid pointer.
         if addr == 0 || global_state.strict_provenance {
             return Pointer::new(None, Size::from_bytes(addr));
         }