X509V3_EXT_add_nconf_sk, X509v3_add_ext: fix errors handling

X509v3_add_ext: free 'sk' if the memory pointed to by it
was malloc-ed inside this function.
X509V3_EXT_add_nconf_sk: return an error if X509v3_add_ext() fails.
This prevents use of a freed memory in do_body:sk_X509_EXTENSION_num().

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4896)