remove tmp0 in update_cdf due to the use of EC_MIN_PROB introduced by
Thomas Davies.

further changes to update_cdf include:
1. Start the rate at 3+get_msb(nsymbs) and increase the rate by one at
counts 16 and 32.
2. Check if tmp is less than cdf[i] to avoid shifting a negative number.

Change-Id: I5088ebd450d6e57ec6c3e92bb2f47a078489b947