splice: Extract remote sig from psbt if needed

On certain well timed restarts we lose their siganture from memory and don’t receive it from them. In these cases we can extract it from the PSBT directly.

channeld/channeld.c

@@ -3420,10 +3420,17 @@ static void resume_splice_negotiation(struct peer *peer,
 		&& have_they_signed_inflight(peer, inflight)) {
 
 		if (!their_sig) {
-			status_failed(STATUS_FAIL_INTERNAL_ERROR,
-				      "Should not arrive here w/o their_sig");
-			return;
+			status_debug("Splice: Extracting their_sig from psbt");
+			if (!psbt_input_get_signature(tmpctx, current_psbt,
+						      splice_funding_index,
+						      &peer->channel->funding_pubkey[REMOTE],
+						      &their_sig))
+				status_failed(STATUS_FAIL_INTERNAL_ERROR,
+					      "Should not arrive here w/o"
+					      " their_sig");
 		}
+		assert(their_sig);
+
 		psbt_input_set_witscript(current_psbt,
 					 splice_funding_index,
 					 wit_script);