lnwallet: fix InternalKeyForAddr for imported addr

An address imported using ImportTapscript doesn't provide a private key
so it can't satisfy waddrmgr.ManagedPubKeyAddress interface. So we don't
return an error for imported addresses.
Fix lightninglabs/loop#923

Author: starius

lnwallet/interface.go

@@ -660,6 +660,12 @@ func InternalKeyForAddr(wallet WalletController, netParams *chaincfg.Params,
 		return none, nil
 	}
 
+	// Imported addresses do not provide private keys, so they do not
+	// implement waddrmgr.ManagedPubKeyAddress. See RPC ImportTapscript.
+	if walletAddr.Imported() {
+		return none, nil
+	}
+
 	pubKeyAddr, ok := walletAddr.(waddrmgr.ManagedPubKeyAddress)
 	if !ok {
 		return none, fmt.Errorf("expected pubkey addr, got %T",