WindowSurface::checkForResize() contained an assert which got
triggered in some tests. In order to avoid this issue, this cl
gracefully handles an invalid window handle.

Change-Id: I6bace88370980e94676d4af9de0d88e561175835
Reviewed-on: https://swiftshader-review.googlesource.com/12808Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>