lib/cache: don't stash packets with zeros in QNAME

lib/cache: don't stash packets with zeros in QNAME

Cache uses dname_lf for keys, i.e. zero bytes serve as separators
between labels.  Therefore having a zero inside label could masquerade
for QNAME that does have label separators instead of these zeros.
That doesn't seem really exploitable in practice, as standard registries
won't allow such labels, so I can't see any possible attack that would
"cross border" of these registries, e.g. attacking anything inside
example.org without any cooperation from its owner (or org or root).