{vp8,vp9}_set_roi_map: fix validation with INT_MIN

previously ranges were checked with abs() whose behavior is undefined
with INT_MIN. this fixes a crash when the original value is returned and
it later used as and offset into a table.

Bug: webm:1742
Change-Id: I345970b75c46699587a4fbc4a059e59277f4c2c8
3 files changed