Fix integer overflow in get_mean_and_dev()
Fix integer overflow in get_mean_and_dev()

BUG=aomedia:1889

Change-Id: Ib7c38689c646563129696b30553ee35a2cead3d5

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