Don't check HEAP in miri.

Author: Berrysoft

library/std/src/sys/pal/windows/alloc.rs

@@ -125,6 +125,8 @@ struct Header(*mut u8);
 unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
     // SAFETY: Check the pointer here.
     let heap = unsafe { get_process_heap() };
+    // Initializer is not called in miri, and it's OK to continue with null.
+    #[cfg(not(miri))]
     if heap.is_null() {
         // Allocation has failed, could not get the current process heap.
         return ptr::null_mut();