Fix compiler warnings in yv12extend.c
Fix compiler warnings in yv12extend.c

Change-Id: I1f6f5b8861c4081b1f4b85c531c5d7ef0cb67bab

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