Remove unnecessary memset(best_index, 0) from trellis/optimize.

First 50 frames of bus @ 1500kbps (speed 0) goes from 2min12.6 to
2min11.6, i.e. 0.75% overall speedup.

Change-Id: I67054f8146e82a02b6457c51a1c8627a937e5e1e