Add null check when mouse coords are bad
Add null check when mouse coords are bad

This happens if the viewport is bad (which was fixed in another commit).

Fixes #801

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