Avoid exiting tx size search when tx-size is square

This fixes a mismatch in ext-tx + rect-tx introduced
by a refactor in 2d147c1603bd666161a46c897cef95c7dabfa49a.

BUG=aomedia:422

Change-Id: I5bc40b2dcd7bf53b20c1bea454aae49f2d5dfe67
1 file changed