cli: fix heap buffer overflow

This patch fixes an invalid read when an empty command was sent.

Found via nft running ASAN and entering an empty command:

nft>

=================================================================
==19540==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000008c6f at pc 0x55e3b561704d bp 0x7fffe9a33ac0 sp 0x7fffe9a33ab8
READ of size 1 at 0x602000008c6f thread T0
    #0 0x55e3b561704c in cli_append_multiline /home/eric/git/netfilter/nftables/src/cli.c:65
    #1 0x55e3b561725b in cli_complete /home/eric/git/netfilter/nftables/src/cli.c:109
    #2 0x7f6e0c2ccac2 in rl_callback_read_char (/lib/x86_64-linux-gnu/libreadline.so.7+0x2fac2)
    #3 0x55e3b5617ba6 in cli_init /home/eric/git/netfilter/nftables/src/cli.c:199
    #4 0x55e3b5573b75 in main /home/eric/git/netfilter/nftables/src/main.c:381
    #5 0x7f6e0bc9b2b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    #6 0x55e3b55725a9 in _start (/usr/local/sbin/nft+0x445a9)

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/cli.c b/src/cli.c
index c1fdcf6c..abb6bf3c 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -61,6 +61,10 @@ static char *cli_append_multiline(char *line)
 	}
 
 	len = strlen(line);
+
+	if (len == 0)
+		return NULL;
+
 	if (line[len - 1] == '\\') {
 		line[len - 1] = '\0';
 		len--;