Flow cycles can occur if we have arc zero arc costs.
The previous path construction from the flow in the network assumed the
absence of such cycles and would enter an infinite loop if it hit one.
With his patch wee add cycle detection and removal during the path
construction phase.
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Lagrang3 <lagrang3@protonmail.com>
Changelog-EXPERIMENTAL: `askrene` infinite loop fixed
bd8cc1fb1f