Update memif_msg_receive_init API socket.c

[amithbraj]

If get_list_elt returns NULL element, then code jumps to error label. Here we are trying to access variable 'c' which is NULL. And 'c' would be set only if get_list_elt succeeds. Also, since lm->control_fd_update() does not use private context, we could either pass NULL or remove the same.