Add a null check to charatlas beginFrame call
Add a null check to charatlas beginFrame call

Fixes #1511

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