Lyrics Quality is NaN and Inf

[wimbledon]

Please reference #5 for more details.

[wimbledon]

@ddddxxx

[wimbledon]

The reason is that similarity func will return Inf if one of the input strings has zero length due to division by zero.

private func similarity(s1: String, s2: String) -> Double {
    let len = min(s1.count, s2.count)
    let diff = min(s1.distance(to: s2, insertionCost: 0), s1.distance(to: s2, deletionCost: 0))
    return Double(len - diff) / Double(len)
}