Rely on Error/Warning message data lengths being correct

In lightning/bolts#950, the (somewhat
strange) requirement that error messages be handled even if the
length field is set larger than the size of the package was
removed. Here we change the code to drop the special handling for
this, opting to just fail to read the message if the length is
incorrect.

lightning/src/ln/msgs.rs

@@ -1524,10 +1524,11 @@ impl Readable for ErrorMessage {
 		Ok(Self {
 			channel_id: Readable::read(r)?,
 			data: {
-				let mut sz: usize = <u16 as Readable>::read(r)? as usize;
-				let data = read_to_end(r)?;
-				sz = cmp::min(data.len(), sz);
-				match String::from_utf8(data[..sz as usize].to_vec()) {
+				let sz: usize = <u16 as Readable>::read(r)? as usize;
+				let mut data = Vec::with_capacity(sz);
+				data.resize(sz, 0);
+				r.read_exact(&mut data)?;
+				match String::from_utf8(data) {
 					Ok(s) => s,
 					Err(_) => return Err(DecodeError::InvalidValue),
 				}
@@ -1550,10 +1551,11 @@ impl Readable for WarningMessage {
 		Ok(Self {
 			channel_id: Readable::read(r)?,
 			data: {
-				let mut sz: usize = <u16 as Readable>::read(r)? as usize;
-				let data = read_to_end(r)?;
-				sz = cmp::min(data.len(), sz);
-				match String::from_utf8(data[..sz as usize].to_vec()) {
+				let sz: usize = <u16 as Readable>::read(r)? as usize;
+				let mut data = Vec::with_capacity(sz);
+				data.resize(sz, 0);
+				r.read_exact(&mut data)?;
+				match String::from_utf8(data) {
 					Ok(s) => s,
 					Err(_) => return Err(DecodeError::InvalidValue),
 				}