Fix debug_assert on our_funding_contribution

When processing a splice_ack, the debug_assert on the range of
our_funding_contribution should account for values that are for both
positive (splice-in) and negative (splice-out).

Author: jkczyz

lightning/src/ln/channel.rs

@@ -11019,7 +11019,7 @@ where
 		};
 
 		let our_funding_contribution = funding_negotiation_context.our_funding_contribution;
-		debug_assert!(our_funding_contribution <= SignedAmount::MAX_MONEY);
+		debug_assert!(our_funding_contribution.abs() <= SignedAmount::MAX_MONEY);
 
 		let their_funding_contribution = SignedAmount::from_sat(msg.funding_contribution_satoshis);
 		self.validate_splice_contribution(their_funding_contribution)?;