tls: fix typo `handle._reading` => `handle.reading`

The problem does not manifest itself on unixes, because
`uv_read_start()` always return 0 there. However on Windows on a second
call `uv_read_start()` returns `UV__EALREADY` destroying all sockets on
a read attempt.

Set `.reading` property that is already handled by `net.js` code.

Fix: https://github.com/iojs/io.js/issues/988
PR-URL: https://github.com/iojs/io.js/pull/994


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>