d3d9: Set IDirect3DDevice9Impl_GetVertexShader return value to NULL on error

[Vincent Pelletier]

When IWineD3DDevice_GetVertexShader fails, set *ppShader to NULL.

This fixes "Black & White 2" here: it used to crash right when first level 
intro video ended, before seeing the level. With this patch I can play a bit 
(though there are some graphical glitches remaining, mouse it not so 
smooth...)

-- 
Vincent Pelletier
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