wined3d: Avoid crashing when ProcessVertices is called with NULL vertex declaration

[Paul Chitescu]

Changelog: Avoid a segfault in IWineD3DDeviceImpl_ProcessVertices by checking 
if SrcImpl (parameter pVertexDecl) is NULL.

The function is called from dlls/d3d8/device.c 
IDirect3DDevice8Impl_ProcessVertices with an explicit NULL as vertex 
declaration parameter.

Crash was observed in Age of Mythology Trial.
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