rule: Avoid segfault with anonymous chains

rule: Avoid segfault with anonymous chains

Phil Sutter says:

"When trying to add a rule which contains an anonymous chain to a
non-existent chain, string_misspell_update() is called with a NULL
string because the anonymous chain has no name. Avoid this by making the
function NULL-pointer tolerant."

Fixes: c330152b7f777 ("src: support for implicit chain bindings")
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>