rpcrt4: Set pStubMsg->BufferMark in NdrConformantVaryingArrayUnmarshall and NdrVaryingArrayUnmarshall.

pStubMsg->BufferMark should always be set before
EmbeddedPointerUnmarshall is called and these functions didn't
previously do so.

To more closely match native format; this is required for windbg.

This hopefully makes the vertex attribute detection simpler, more solid, 
readable and flexible.

Varying arrays that aren't unmarshalled shouldn't be freed, but 
conformant arrays that aren't unmarshalled should.

Also print an error message if there was an overflow.