Avoid underflow on an unsigned int when computering bg overrun

Avoid underflow on an unsigned int when computering bg overrun

A computation for when a block group overruns the end of an image
was consistently storing a negative number in an unsigned var,
and then checking for >0, which is always true for negative numbers.
So first check if the number will be positive before computing it.

Change-Id: Ic8cff8f9ab9f4ea8c5a4dc42143c2430fa87ba12