Fixes a failure with probetest:
  ./tools/probetest 8192 500000
  [...]
  Failure of jacosub probing code with score=51 type=1 p=CAB size=16

Random data was:
  00000000  40 37 40 30 93 01 48 e0 09 96 10 90 78 0f 0c 10 @7@0..H.....x...