Check if socket is already running

instant-markdown would never start (though it worked before).
After changing "/dev/null" to "log", I saw the following error:

instant-markdown-d/instant-markdown-d:62
        socket.emit('newContent', newHTML);
               ^
TypeError: Cannot call method 'emit' of undefined

After inserting `if (socket)` before `socket.emit`, the vim
plugin was working again.