smb: prevents integer underflow

Ticket: 5246

If msg_id is 0, we cannot find the previous request

rust/src/smb/smb2.rs

@@ -376,10 +376,14 @@ pub fn smb2_request_record<'b>(state: &mut SMBState, r: &Smb2Record<'b>)
                                 None => {
                                     // try to find latest created file in case of chained commands
                                     let mut guid_key = SMBCommonHdr::from2(r, SMBHDR_TYPE_FILENAME);
-                                    guid_key.msg_id = guid_key.msg_id - 1;
-                                    match state.ssn2vec_map.get(&guid_key) {
-                                        Some(n) => { n.to_vec() },
-                                        None => { b"<unknown>".to_vec()},
+                                    if guid_key.msg_id == 0 {
+                                        b"<unknown>".to_vec()
+                                    } else {
+                                        guid_key.msg_id = guid_key.msg_id - 1;
+                                        match state.ssn2vec_map.get(&guid_key) {
+                                            Some(n) => { n.to_vec() },
+                                            None => { b"<unknown>".to_vec()},
+                                        }
                                     }
                                 },
                             };