ole32: Add a NULL pointer check in CoDisconnectObject.

dlls/ole32/compobj.c

@@ -1557,6 +1557,8 @@ HRESULT WINAPI CoDisconnectObject( LPUNKNOWN lpUnk, DWORD reserved )
 
     TRACE("(%p, 0x%08x)\n", lpUnk, reserved);
 
+    if (!lpUnk) return E_INVALIDARG;
+
     hr = IUnknown_QueryInterface(lpUnk, &IID_IMarshal, (void **)&marshal);
     if (hr == S_OK)
     {

dlls/ole32/tests/marshal.c

@@ -1331,6 +1331,9 @@ static void test_disconnect_stub(void)
     CoDisconnectObject((IUnknown*)&Test_ClassFactory, 0);
 
     ok_no_locks();
+
+    hr = CoDisconnectObject(NULL, 0);
+    ok( hr == E_INVALIDARG, "wrong status %x\n", hr );
 }
 
 /* tests failure case of a same-thread marshal and unmarshal twice */