Fix signed and unsigned integer comparison issue
Fix signed and unsigned integer comparison issue

Fixed signed and unsigned integer comparison issue.

BUG= aomedia:2129

Change-Id: I46d19d99806d72ac38f74802fa2c6e450eaa961a

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