Fix a panic that can occur when the window size is zero.
Fix a panic that can occur when the window size is zero.

Early exiting here can result in the aux. lists getting out of
sync with the display list.

In the future, we can revisit this and early out. However, this
is an immediate fix for a minimize crash on Windows.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender