Call WriteFile even for empty messages

This is necessary to avoid hanging in the corresponding ReadFile calls.
Before we kept the engine connections open, this just caused threads to hang
indefinitely. Now that we're using just one connection, it causes the whole tab to hang indefinitely.

[Wladimir Palant, 2013-08-13 10:17:20]

LGTM, let's hope that this actually works correctly in all cases. From
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365747%28v=vs.85%29...:

"A value of zero specifies a null write operation. The behavior of a null write
operation depends on the underlying file system or communications technology."

[Felix Dahlke, 2013-08-13 10:21:39]

On 2013/08/13 10:17:20, Wladimir Palant wrote:
> LGTM, let's hope that this actually works correctly in all cases. From
>
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365747%2528v=vs.85%...:
> 
> "A value of zero specifies a null write operation. The behavior of a null
write
> operation depends on the underlying file system or communications technology."

I've tested it quite a bit on this system, but I'll upload a new devbuild with
this and test it on some other configurations just in case. I would be quite
surprised if this is handled differently on different systems though.

[Oleksandr, 2013-08-13 12:00:25]

LGTM as well.