Scoring performance enhancements.

The old scoring code computed the afsi, afhi, and the path of the audio file for each
item of the mood,  which is unnecessary.  This patch moves these computations from
get_item_score() into compute_mood_score() so that afsi, afhi and path are only
computed once  per audio file.

As a result, get_item_score() can no longer fail, so we may skip the checks for negative
return values.