netfilter: nfnetlink_acct: Fix memory leak

Allocation of memory need only to happen once, that is
after the proper checks on the NFACCT_FLAGS have been
done.  Otherwise the code can return without freeing
already allocated memory.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>