stream: fix memory leak when failing to insert segment

As introduced by commit f848e34bcc

DoInsertSegment can now return `-EINVAL` and there was no
generic handling of error values to return the tcp segment
to its pool.

Ticket: #5777

src/stream-tcp-list.c

@@ -703,6 +703,9 @@ int StreamTcpReassembleInsertSegment(ThreadVars *tv, TcpReassemblyThreadCtx *ra_
             }
 #endif
         }
+    } else {
+        // EINVAL
+        StreamTcpSegmentReturntoPool(seg);
     }
 
     SCReturnInt(0);