Validate negative splice_init.funding_contribution_satoshis

From BOLT #2:
```
- If `funding_contribution_satoshis` is negative and its absolute value
  is greater than the sending node's current channel balance:
  - MUST send a `warning` and close the connection or send an `error`
    and fail the channel.
```

We allow the remote to be below the new reserve as long as their
funding contribution is not negative; we don't care whether they were
above or below the previous funding reserve.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -10748,7 +10748,9 @@ where
 		// Note on channel reserve requirement pre-check: as the splice acceptor does not contribute,
 		// it can't go below reserve, therefore no pre-check is done here.
 
-		// TODO(splicing): Early check for reserve requirement
+		if their_funding_contribution_satoshis.is_negative() {
+			self.validate_their_negative_funding_contribution(&splice_funding)?;
+		}
 
 		Ok(splice_funding)
 	}