Cluster: Fix segfault if cluster config corrupt

This commit adds a size check after initial config
line parsing to make sure we have *at least* 8 arguments
per line.

If someone has garbage in their cluster-config-file,
Redis will now exit with an error message instead of
segfaulting due to accessing argv[N] when only argv[N-1]
exists.

The segfault is because Redis expects at least 8 arguments
per line (extra arguments get added when slots are
assigned), and the config parser statically accesses
argv[0] through argv[7] (and optionally argv[8+] when
slots are assigned).

This commit does introduce a static number in the form of
"if argc<8", but the magic number 8 exists in the
form of j=8 in the slot assignment parsing loop too.