smb: prevents integer underflow

Ticket: 5246

If msg_id is 0, we cannot find the previous request

rust/src/smb/smb2.rs

@@ -376,11 +376,15 @@ 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);
+                                    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()},
                                         }
+                                    }
                                 },
                             };
                             let tx = state.new_setfileinfo_tx(fname, rd.guid.to_vec(), rd.class as u16, rd.infolvl as u16, dis.delete);