Pass a temporary pointer to o2i_SCT_signature from SCT_new_from_base64

Otherwise, |dec| gets moved past the end of the signature by
o2i_SCT_signature and then can't be correctly freed afterwards.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1548)