Fix for AV1.TestTell
Fix for AV1.TestTell

The tell functions return an unsigned integer.
This causes the AV1.TestTell test case to fail because
-1 is greater than 20 when treated as an unsigned integer.

Change-Id: I9dd1d7eb61260d30d1713a4917159fc6fe8eee42

mbebenita/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,smarter/aom,mbebenita/aom,smarter/aom