add memset to allocated buffers
add memset to allocated buffers

BUG=aomedia:1419

Change-Id: I66927c95e81ee488446c7f10abb5d5c8f89d155c

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