internal/poll: handle zero-byte write in FD.WriteTo

Zero-byte write was fixed by CL 132781, that was submitted 3 days ago.
But I just submitted CL 129137, and the CL broken zero-byte write
functionality without me noticing. CL 129137 was based on old commit
(older than 3 days ago), and try-bots did not discover the breakage.

Fix zero-byte write again.

Fixes windows build.

Change-Id: Ib403b25fd25cb881963f25706eecca92b924aaa1
Reviewed-on: https://go-review.googlesource.com/134275


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>