Fix assert failure with tx64x64 and ext-partiton
Fix assert failure with tx64x64 and ext-partiton

Fixes an assert failure when 32x64 and 64x32 partitions are used
with tx64x64 when ext-partition is also on.

BUG=aomedia:935

Change-Id: I3bf50aeab58ec4ba2c9892e6eb18cf60e425fa42

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