lnpeer: don't save our own channel update as remote upd

I noticed CLN is sending our own channel update to us on
reestablishment, we then assume it to be the remote nodes
update and try to verify the signature against their pubkey
which fails and throws `InvalidGossipMsg`.

This adds a check preventing us from trying to save our own
channel updates as remote update.

electrum/lnpeer.py

@@ -55,6 +55,7 @@ from .interface import GracefulDisconnect
 from .json_db import StoredDict
 from .invoices import PR_PAID
 from .fee_policy import FEE_LN_ETA_TARGET, FEERATE_PER_KW_MIN_RELAY_LIGHTNING
+from .channel_db import FLAG_DIRECTION
 
 if TYPE_CHECKING:
     from .lnworker import LNGossip, LNWallet
@@ -470,6 +471,12 @@ class Peer(Logger, EventListener):
             return
         for chan in self.channels.values():
             if payload['short_channel_id'] in [chan.short_channel_id, chan.get_local_scid_alias()]:
+                # originator: node_id_1 if the least-significant bit of flags is 0 or node_id_2 otherwise
+                flags = int.from_bytes(payload['channel_flags'], byteorder='big', signed=False)
+                originator = sorted(self.node_ids)[flags & FLAG_DIRECTION]
+                if originator == self.lnworker.node_keypair.pubkey:
+                    self.logger.debug(f"peer sent us our own channel update for chan {chan.get_id_for_log()}")
+                    return
                 chan.set_remote_update(payload)
                 self.logger.info(f"saved remote channel_update gossip msg for chan {chan.get_id_for_log()}")
                 break