use is_null_ptr instead of to_bits_or_ptr

Author: RalfJung

src/shims/foreign_items.rs

@@ -637,12 +637,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let key_ptr = this.read_scalar(args[0])?.not_undef()?;
 
                 // Extract the function type out of the signature (that seems easier than constructing it ourselves).
-                let dtor = match this.read_scalar(args[1])?.not_undef()?.to_bits_or_ptr(this.memory().pointer_size(), tcx) {
-                    Err(dtor_ptr) =>
-                        Some(this.memory().get_fn(dtor_ptr.into())?.as_instance()?),
-                    Ok(0) => // NULL pointer
-                        None,
-                    Ok(_) => return err!(ReadBytesAsPointer),
+                let dtor = match this.read_scalar(args[1])?.not_undef()?;
+                let dtor = if dtor.is_null_ptr() {
+                    Some(this.memory().get_fn(dtor_ptr)?.as_instance()?)
+                } else {
+                    None
                 };
 
                 // Figure out how large a pthread TLS key actually is.