Add null check when fetching line string
Add null check when fetching line string

See Microsoft/vscode#34466

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