Small code cleaning and partially resolves issue 1171. I can't confirm
that it's a complete fix because the code asserts further a long

aomenc: ../av1/common/blockd.h:1064: av1_get_tx_type:
  Assertion `tx_type < TX_TYPES' failed.

BUG=aomedia:1171

Change-Id: I3f1c91d751e937dd8e07f7e2f786a75eba4b6891