redis-info.nse breaks when parsing client connections response

[franklymrshankley]

Describe the bug
When running the redis-info.nse script, the script appears to handle empty / null values in the "client connections" responses in a less than graceful manner, and exits with an error and a stack trace. This prevents any other info obtained from the redis server from being displayed.

The following is displayed with -d flag enabled.

NSE: redis-info against 127.0.0.1:6379 threw an error!
/usr/local/bin/../share/nmap/scripts/redis-info.nse:142: table index is nil
stack traceback:
	/usr/local/bin/../share/nmap/scripts/redis-info.nse:142: in local 'process'
	/usr/local/bin/../share/nmap/scripts/redis-info.nse:254: in function </usr/local/bin/../share/nmap/scripts/redis-info.nse:185>
	(...tail calls...)

The problem appears to occur when handling responses to the CLIENT LIST command where the results table contains a blank line, and manifests on line 139 which sets the client IP in the array (client_ips[ip] = true;).

I have debugged and resolved locally by adding the following check just inside the for loop which contains the failing line:

        if not item or 0 == #item then break end

but I accept that there may be more elegant ways to address this.

I have tested this with the redis 2.6 and 6.2.2, and get the same results across both versions.

To Reproduce

$ docker run --rm -ti --name test-redis-info-nse -p 6379:6379 redis
$ nmap -sSV -vv -n -d -p 6379 --script redis-info.nse 127.0.0.1

Expected behavior
Where the redis server allows access, then information should be displayed relating to the server, e.g.

PORT     STATE SERVICE REASON         VERSION
6379/tcp open  redis   syn-ack ttl 64 Redis key-value store 2.6.17 (64 bits)
| redis-info: 
|   Version: 2.6.17
|   Operating System: Linux 3.1.0-28-generic x86_64
|   Architecture: 64 bits
|   Process ID: 1
|   Used CPU (sys): 1.18
|   Used CPU (user): 0.85
|   Connected clients: 28
|   Connected slaves: 0
|   Used memory: 836.97K
|   Role: master
|   Bind addresses: 
|     0.0.0.0
|   Client connections: 
|_    172.17.0.1
Final times for host: srtt: 100 rttvar: 5000  to: 100000

Version info (please complete the following information):

• OS: linux, various
• Output of nmap --version:

Nmap version 7.91 ( https://nmap.org )
Platform: x86_64-unknown-linux-gnu
Compiled with: nmap-liblua-5.3.5 openssl-1.1.1f libssh2-1.8.0 libz-1.2.11 libpcre-8.39 libpcap-1.9.1 nmap-libdnet-1.12 ipv6
Compiled without:
Available nsock engines: epoll poll select

Additional context
None.

[nnposter]

Thank you both for reporting the issue and proposing a fix. A more comprehensive code refactoring was committed as r38378.