AP_Scripting: prevent re-use of closed socket

this prevents a null pointer dereference after a close of a network
socket

libraries/AP_Scripting/lua_bindings.cpp

@@ -826,6 +826,7 @@ int SocketAPM_close(lua_State *L) {
             ud->close();
             delete ud;
             scripting->_net_sockets[i] = nullptr;
+            *check_SocketAPM(L, 1) = nullptr;
             break;
         }
     }