mnl: silence compiler warning

mnl: silence compiler warning

gcc 14.3.0 reports this:

src/mnl.c: In function 'mnl_nft_chain_add':
src/mnl.c:916:25: warning: 'nest' may be used uninitialized [-Wmaybe-uninitialized]
  916 |                         mnl_attr_nest_end(nlh, nest);

I guess its because compiler can't know that the conditions cannot change
in-between and assumes nest_end() can be called without nest_start().

Fixes: 01277922fede ("src: ensure chain policy evaluation when specified")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>