Handle NULL from 'ServerSocketChannel#accept'

If a ServerSocketChannel is set into non-blocking mode, its 'accept'
method returns NULL if there are no pending connections. As noted in
SelectionKey documentation, having an operation in the ready set of
a selection key does not guarantee that the operation could be
performed in non-blocking manner.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #41 from jvirtanen/fixes/non-blocking-accept

Handle NULL from 'ServerSocketChannel#accept'