accounting_test: fix read of uninitialized data
accounting_test: fix read of uninitialized data

Only read bits that were actually written.

Change-Id: I6d901123c319a1d92c54f511d3caa56daf882281

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