Use sk_CONF_VALUE_pop_free in do_ext_nconf error path.

8605abf13523579ecab8b1f2a4bcb8354d94af79 fixed the nval leak, but it
used free instead of pop_free. nval owns its contents, so it should be
freed with pop_free. See the pop_free call a few lines down.

This is a no-op as, in this codepath, we must have nval == NULL or
sk_CONF_VALUE_num(nval) == 0. In those cases, free and pop_free are
identical. However, variables should be freed consistently.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1351)