Bugfix of signed integer overflow
Bugfix of signed integer overflow

BUG=aomedia:2009

Change-Id: Ic05f772d4c9c469cf30afc5ff67de4438b0ae112

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom