the max is 6. there are assumptions throughout the decode regarding
this; fixes a crash with a fuzzed bitstream

$ zzuf -s 5861 -r 0.01:0.05 -b 6- \
  < vp90-2-00-quantizer-00.webm.ivf \
  | dd of=invalid-vp90-2-00-quantizer-00.webm.ivf.s5861_r01-05_b6-.ivf \
    bs=1 count=81883

Change-Id: I6af41bb34252e88bc156a4c27c80d505d45f5642