ntdll: Tolerate null handle in DeregisterWait.

dlls/kernel32/tests/thread.c

@@ -1245,6 +1245,10 @@ static void test_RegisterWaitForSingleObject(void)
 
     ret = pUnregisterWait(wait_handle);
     ok(ret, "UnregisterWait failed with error %d\n", GetLastError());
+
+    ret = pUnregisterWait(0);
+    ok(!ret && GetLastError() == ERROR_INVALID_HANDLE,
+        "UnregisterWait failed with unexpected error %d\n", GetLastError());
 }
 
 static DWORD LS_main;

dlls/ntdll/threadpool.c

@@ -637,6 +637,9 @@ NTSTATUS WINAPI RtlDeregisterWaitEx(HANDLE WaitHandle, HANDLE CompletionEvent)
 
     TRACE( "(%p)\n", WaitHandle );
 
+    if (WaitHandle == NULL)
+        return STATUS_INVALID_HANDLE;
+
     NtSetEvent( wait_work_item->CancelEvent, NULL );
     if (wait_work_item->CallbackInProgress)
     {