In some situations console read callback would return 0 bytes read.
According to documentation this means EAGAIN or EWOULDBLOCK, which is
not the case in any of the situations that currently happen.
This removes those zero-size callbacks.
Fixes: https://github.com/libuv/libuv/issues/2012
PR-URL: https://github.com/libuv/libuv/pull/2014
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>