Better fix for stack corruption issue on windows

When the user closes an uv_poll_t handle, there may still be an
outstanding AFD_POLL request. Libuv can't cancel this AFD_POLL request
directly, so it has to submit another which effectively the previous
one.

Libuv completely forgets about this "canceling" AFD_POLL request, but
at some point in time it may complete and store its result somewhere in
memory.

Without this patch the result of the canceling poll request is written
somewhere on the stack, which sometimes causes stack corruption. This
patch ensures that the result is written to some static scratch space
where it can do no harm.

R=@saghul
R=@schlamar
That memory only ever gets written but is never read.