abstract_tcp_server2: add missing return

(cherry picked from commit f60b400917a6d2fecc6c03524c70fbd9e033f436)

contrib/epee/include/net/abstract_tcp_server2.inl

@@ -516,6 +516,7 @@ namespace net_utils
             start_write();
           }
         });
+        return;
       }
     }