lib-master: Fixed memory leak when IPC server is handling commands.

This mainly meant that when login processes responded to doveadm proxy
list/kick commands memory was leaked.

src/lib-master/ipc-server.c

@@ -175,6 +175,7 @@ static void ipc_cmd_finish(struct ipc_cmd *cmd, const char *line)
 
 	i_assert(cmd->server->ipc_cmd_refcount > 0);
 	cmd->server->ipc_cmd_refcount--;
+	i_free(cmd);
 }
 
 void ipc_cmd_success(struct ipc_cmd **_cmd)