netfilter: nft_log: fix possible memory leak if log expr init fail

Suppose that we specify the NFTA_LOG_PREFIX, then NFTA_LOG_LEVEL
and NFTA_LOG_GROUP are specified together or nf_logger_find_get
call returns fail, i.e. expr init fail, memory leak will happen.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>