cli: fix nft -i command crashes when try to input multi line command

cli: fix nft -i command crashes when try to input multi line command

When try to input multiline command in "nft -i", it crashes.

Issue is, function cli_append_multiline() return null in case of
multiline command. But in the calling function cli_complete(),
cli_exit is getting called, which in turn calls
rl_callback_handler_remove() and the handler is getting removed.

 [root@localhost ~]# nft -i
 nft> add table filter
 nft> list table \

 readline: readline_callback_read_char() called with no handler!
 Aborted (core dumped)
 [root@localhost ~]#

After this patch, it shows:

 nft> list table \
 .... filter
 table ip filter {
 }
 nft>

The ".... " prompt is used to indicate a multiline command, similar to
what Python does.

Signed-off-by: Guruswamy Basavaiah <guru2018@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>