netfilter: x_tables: handle xt_register_template() returning an error value

Commit fdacd57c ("netfilter: x_tables: never register tables by
default") introduces the function xt_register_template(), and in one case,
a call to that function was missing the error-case handling.

Handle when xt_register_template() returns an error value.

This was identified with the clang-analyzer's Dead-Store analysis.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>