NtGdiCreateHalftonePalette should not attempt lock if HDC is null

(from Mattermost chat)
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mshtml.dll calls shlwapi's CreateHalftonePalette(NULL);, that calls win32's NtGdiCreateHalftonePalette(NULL) - https://github.com/reactos/reactos/blob/master/win32ss/gdi/ntgdi/palette.c#L526
It tries to lock the null (and unused along all the function body) HDC, fails and returns ERROR_INVALID_HANDLE;
That seems to abort the loading of the dll - wine (and potentially Windows) does not try to loc the null HDC.

With the check, the DLL loads successfully.