Don't rely on `calculate_success_probability*` to handle amt > cap

Don't rely on `calculate_success_probability*` to handle amt > cap

Currently we let an `htlc_amount >= channel_capacity` pass through
from `penalty_msat` to
`calculate_success_probability_times_billion`, but only if its only
marginally bigger (less than 65/64ths). This is fine as
`calculate_success_probability_times_billion` handles bogus values
just fine (it will always return a zero probability in such cases).

However, this is risky, and in fact breaks in the coming commits,
so instead check it before ever calling through to the historical
bucket probability calculations.