f: use LengthReadable in missed instance

Author: arik-so

lightning/src/util/ser_macros.rs

@@ -453,7 +453,7 @@ macro_rules! _decode_tlv {
 	}};
 	($outer_reader: expr, $reader: expr, $field: ident, (option, encoding: ($fieldty: ty, $encoding: ident))) => {{
 		$field = {
-			let field: $encoding<$fieldty> = ser::Readable::read(&mut $reader)?;
+			let field: $encoding<$fieldty> = ser::LengthReadable::read(&mut $reader)?;
 			Some(field.0)
 		};
 	}};