Fix overflow in ProbabilisticScorer

Fix overflow in ProbabilisticScorer

When a routing hint is given in an invoice, the effective capacity of
the channel is assumed to be infinite (i.e., u64::max_value) if the hop
is private. Adding 1 to this in the success probability calculation will
cause an overflow and ultimately an `index out of bounds panic` in
log10_times_1024. This was not an issue with using log10 because the use
of f64 would give infinite which casts to 0 for u64.