ssh/frames: avoid unsigned integer overflow

Fixes: 0b2ed97f36 ("ssh: frames support")

rust/src/ssh/ssh.rs

@@ -233,7 +233,8 @@ impl SSHState {
                                 flow,
                                 stream_slice,
                                 input,
-                                (head.pkt_len + 4) as i64,
+                                // cast first to avoid unsigned integer overflow
+                                (head.pkt_len as u64 + 4) as i64,
                                 SshFrameType::RecordPdu as u8,
                                 Some(0),
                             );