Auto merge of #1788 - hyd-dev:rustup, r=RalfJung

`encountered a NULL reference` -> `encountered a null reference`

It's changed from "NULL" to "null" (probably by rust-lang/rust#84842) in `rustc`, and causing some test failures:
https://github.com/rust-lang/miri/runs/2498333632#step:8:640

rust-version

@@ -1 +1 @@
-59f551a2dcf57c0d3d96ac5ef60e000524210469
+0309953232d9957aef4c7c5a24fcb30735b2066b

tests/compile-fail/validity/cast_fn_ptr1.rs

@@ -7,5 +7,5 @@ fn main() {
     let g: fn(*const i32) = unsafe { std::mem::transmute(f as fn(&i32)) };
 
     g(0usize as *const i32)
-    //~^ ERROR encountered a NULL reference
+    //~^ ERROR encountered a null reference
 }

tests/compile-fail/validity/cast_fn_ptr2.rs

@@ -7,5 +7,5 @@ fn main() {
     let g: fn() -> &'static i32 = unsafe { std::mem::transmute(f as fn() -> *const i32) };
 
     let _x = g();
-    //~^ ERROR encountered a NULL reference
+    //~^ ERROR encountered a null reference
 }