Fix memory leak in case of recvfrom(2) failure in Lua sockets

Thanks Alexey Antipovsky

Closes #1448

src/lua/socket.c

@@ -859,6 +859,7 @@ lbox_socket_recvfrom(struct lua_State *L)
 			       (struct sockaddr*)&fa, &len);
 
 	if (res < 0) {
+		free(buf);
 		lua_pushnil(L);
 		return 1;
 	}