netfilter: nf_tables: fix chain filter in nf_tables_dump_rules()

ctx->chain may be null now that we have very large object names,
so we cannot check for ctx->chain[0] here.

Fixes: b7263e07

 ("netfilter: nf_tables: Allow table names of up to 255 chars")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Phil Sutter <phil@nwl.cc>