Fix ubsan warning: md5_utils.c
Fix ubsan warning: md5_utils.c

This commit ports a change from libvpx to prevent ubsan warnings.

Change-Id: I4c7688c34eb5ecdcb1377d8afeef734072bdccc1

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