add another test for #2068

Author: RalfJung

tests/run-pass/issue-miri-2068-2.rs

@@ -0,0 +1,14 @@
+// compile-flags: -Zmiri-disable-validation
+
+use std::mem::MaybeUninit;
+
+fn main() { unsafe {
+    let mut x = MaybeUninit::<i64>::uninit();
+    // Put in a ptr.
+    x.as_mut_ptr().cast::<&i32>().write_unaligned(&0);
+    // Overwrite parts of that pointer with 'uninit' through a Scalar.
+    let ptr = x.as_mut_ptr().cast::<i32>();
+    *ptr = MaybeUninit::uninit().assume_init();
+    // Reading this back should hence work fine.
+    let _c = *ptr;
+} }