Guard usages of DOM attached vars in setOption
Guard usages of DOM attached vars in setOption

Fixes #1383

Tyriar/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js