fix(prost-codec): checked addition in Codec::decode

If the length of the varint at the beginning of a decoded value plus the payload length reported within it overflows, we can cheat the test for the remaining buffer length, and the `src.split_to(message_length)` call will then cause a panic as `message_length` is a very large value.

Pull-Request: #6467.

Author: dknopik

misc/prost-codec/src/lib.rs

@@ -81,7 +81,10 @@ where
         let varint_length = src.len() - remaining.len();
 
         // Ensure we can read an entire message.
-        if src.len() < (message_length + varint_length) {
+        if message_length
+            .checked_add(varint_length)
+            .is_none_or(|total_length| src.len() < total_length)
+        {
             return Ok(None);
         }