Change order of operations to ensure that askrene-inform-channel with
inform=constrained computes the correct upper bound on the liquidity:
upperBound = reserves + amount - 1
With the previous order of operations in the case amount=0 we would get:
upperBound = reserves
instead of
upperBound = reserves - 1
Changelog-None
Signed-off-by: Lagrang3 <lagrang3@protonmail.com>
81417958d4