netapi32: Set the buffer to NULL in NetApiBufferFree

[Juan Lang]

> Not that I know anything but it seems that HeapFree calls RtlFreeHeap
> and RtlFreeHeap checks to make sure the pointer is not null before
> trying to free it.  So explicitly setting it to null makes the erorr
> go away.

That might be true if setting Buffer to NULL happened before the call
to HeapFree, but it happens after.  After the return from
NetApiBufferFree, Buffer goes out of scope, and a subsequent call
won't be affected by it.

So again, this seems like a Valgrind bug.  How is the warning getting
silenced by this assignment?
--Juan