Clear interactive_tx_signing_session upon splice promotion

Once splice has been promoted, the interactive_tx_signing_session is no
longer needed. Clear it at this time to prevent being in an inconsistent
state.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -6085,6 +6085,7 @@ macro_rules! promote_splice_funding {
 			$self.context.historical_scids.push(scid);
 		}
 		core::mem::swap(&mut $self.funding, $funding);
+		$self.interactive_tx_signing_session = None;
 		$self.pending_splice = None;
 		$self.pending_funding.clear();
 		$self.context.announcement_sigs_state = AnnouncementSigsState::NotSent;