lib: fix error code from nfnl_exp_build_message()

Otherwise we return success but don't actually set the output
result. This can lead to a crash, in case of out-of-memory.

Found by Coverity.

https://bugzilla.redhat.com/show_bug.cgi?id=1606988

lib/netfilter/exp.c

@@ -492,6 +492,8 @@ static int nfnl_exp_build_message(const struct nfnl_exp *exp, int cmd, int flags
 	return 0;
 
 nla_put_failure:
+	err = -NLE_NOMEM;
+
 err_out:
 	nlmsg_free(msg);
 	return err;