Fix scorer panic when available capacity is zero

Fix scorer panic when available capacity is zero

ProbabilisticScorer takes a ChannelUsage when computing a penalty for a
channel. The formula for calculating the liquidity penalty reduces the
maximum capacity by the amount of in-flight HTLCs (available capacity)
and adds one to prevent division by zero.

However, since the available capacity is passed to
DirectedChannelLiquidity as the capacity, other penalty formulas may use
the available (i.e., reduced) capacity inadvertently. In practice, this
has two ramifications for the historical liquidity penalty computation:

1. The bucket formula doesn't have a consistent denominator for a given
   channel.
2. The bucket formula may divide by zero when the in-flight HTLC amount
   equals or exceeds the effective capacity.

Fixing this involves only using the available capacity when appropriate.