rpcrt4: Improve PointerUnmarshall to cope with keeping the non-NULL source pointer of a client unmarshall.

This will enable various types with embedded pointers to handle [in,out] 
parameters correctly by keeping the existing memory.

This will enable them to be individually fixed up to use memory if it 
was provided by the caller.

rpcrt4: EmbeddedPointerUnmarshall doesn't need to change the address of the allocated memory, so reduce the level of indirection of the memory parameter by one.

rpcrt4: Errors in sending or receiving packets should result in RPC_S_CALL_FAILED being returned, not RPC_S_PROTOCOL_ERROR.