cache: initialize filter when fetching implicit chains

cache: initialize filter when fetching implicit chains

ASAN reports:

  src/cache.c:734:25: runtime error: load of value 189, which is not a valid value for type '_Bool'

because filter->reset.rule remains uninitialized.

Initialize filter and replace existing construct to initialize table and
chain which leaves remaining fields uninitialized.

Fixes: dbff26bfba83 ("cache: consolidate reset command")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>