Fix event loop io tree inconsistency on Windows.

Fix event loop io tree inconsistency on Windows.

On Windows, the event loop io tree uses the Windows Event handle to
differentiate between io_t objects. Unfortunately, there is a bug in
the io_add_event() function (introduced in
2f9a1d4ab5ff51b05a5e8cc41a1528fdeb36c723) as it sets the event after
inserting the object into the tree, resulting in objects appearing in
io_tree out of order.

This can lead to crashes on Windows as the event loop is unable to
determine which events fired.