netlink-socket: Avoid use-after-free in nl_lookup_genl_mcgroup().

netlink-socket: Avoid use-after-free in nl_lookup_genl_mcgroup().

Commit e408762f "netlink-socket: New function nl_lookup_genl_mcgroup()"
modified do_lookup_genl_family() to return the Netlink attributes to the
caller, but it still freed the Netlink message itself, which meant that
the attributes pointed into freed memory.  This commit fixes the problem.

This commit is not a minimal fix.  It refactors do_lookup_genl_family(),
changing the return value from "negative errno value or positive genl
family id" to the more common "zero or positive errno value".

Found by valgrind.