obu: Fix decoder_peek_si_interal for size_after_header
obu: Fix decoder_peek_si_interal for size_after_header

BUG=aomedia:1531

Change-Id: Id0f4bb827e51c1e70b42eb57ae6e1075274a217c

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