Even though number of unique colors in a block may be at least
PALETTE_MIN_SIZE, sometimes the K-means can generate fewer than
PALETTE_MIN_SIZE unique centroids due to integer rounding.

This was resulting in "k - PALETTE_MIN_SIZE" being negative and using
that as an index into palette_y_size_cost array was causing a UBSan
error.

BUG=aomedia:515

Change-Id: Ia00b4bb2efebbe43d1fec7b619ebb837b1ae961c