Fix for 118632. file picker can display all column arrows.
r=bryner, sr=hyatt
- getCellAt() now checks for correct mouse coordinates and sets row value
to -1 for invalid coordinates.
- added bail out checks to all callers.
git-svn-id: svn://10.0.0.236/trunk@111793 18797224-902f-48f8-a5cc-f745e15eee43