Fix affected MIRI tests by disabling the checks for them

Author: 1c3t3a

src/tools/miri/tests/fail/dangling_pointers/null_pointer_deref.rs

@@ -1,3 +1,6 @@
+// Enable the UB checks for the sake of this test.
+//@compile-flags: -Zub-checks=no
+
 #[allow(deref_nullptr)]
 fn main() {
     let x: i32 = unsafe { *std::ptr::null() }; //~ ERROR: null pointer

src/tools/miri/tests/fail/dangling_pointers/null_pointer_write.rs

@@ -1,3 +1,6 @@
+// Enable the UB checks for the sake of this test.
+//@compile-flags: -Zub-checks=no
+
 #[allow(deref_nullptr)]
 fn main() {
     unsafe { *std::ptr::null_mut() = 0i32 }; //~ ERROR: null pointer