Fix build with --enable-obu
Fix build with --enable-obu

It seems that the change in commit 28444be missed this line (which
only fails to compile with --enable-debug because it's in an assert)

BUG=aomedia:952

Change-Id: Ibe6200b2928d198c73f3e9c9f0badcbdaffcac49

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