Convert "var" to uint64_t
Convert "var" to uint64_t

This is to avoid overflow at uint32_t.

cherry-picked #000098a0 from aom/master

Change-Id: I549d2d13d0577fd05d57303a438fbc8034755e45

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