d3d9: Set IDirect3DDevice9Impl_GetVertexShader return value to NULL on error

[Stefan Dösinger]

> No, this patch is obviously correct.
Hmm right, the code treats pShader == NULL as 'failure'. Looking at the WineD3D code, it would only fail if ppShader == NULL, which would moot the point of setting *ppShader to NULL anyway.

My bad in this case