Fixed integer overflow in overview for large line numbers and large over...

I noticed a render error in the Overview on my machine.
I happen to have a 1920 vertical monitor and was viewing a 15 million line file, and the overview was only showing up in the top 15% of area and then as it filtered through file I could see sections of file overwriting that first 15%.

I believe this addresses the integer overflow that was occurring.

(cherry picked from commit 2e6412d21fc307b7b9879005275611acd65bc1d8)