Fix assertion failure in var_tx+rect_tx-ext_tx
Fix assertion failure in var_tx+rect_tx-ext_tx

BUG=aomedia:560

Change-Id: Ie399d61162ab33e35fa0cb04206904e3af1bbb2d

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