idct.c: reduce scope of iterators
idct.c: reduce scope of iterators

Change-Id: Ia902c4d8ba61d98d2ee3165daed90a4a70476177

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom