Set uv's custom data before uv_read_start

This is a behavioral difference in libuv between different platforms in
different situations. It turns out that libuv on windows will immediately
allocate a buffer instead of waiting for data to be ready. What this implies is
that we must have our custom data set on the handle before we call
uv_read_start.

I wish I knew of a way to test this, but this relies to being on the windows
platform *and* reading from a true TTY handle which only happens when this is
actually attached to a terminal. I have manually verified this works.

Closes #10645
auto merge of #10646 : alexcrichton/rust/issue-10645, r=luqmana

This is a behavioral difference in libuv between different platforms in
different situations. It turns out that libuv on windows will immediately
allocate a buffer instead of waiting for data to be ready. What this implies is
that we must have our custom data set on the handle before we call
uv_read_start.

I wish I knew of a way to test this, but this relies to being on the windows
platform *and* reading from a true TTY handle which only happens when this is
actually attached to a terminal. I have manually verified this works.

Closes #10645