Fix a crash in reuse of i2d_X509_PUBKEY

If the second PUBKEY is malformed there is use after free.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8122)

(cherry picked from commit 5dc40a83c74be579575a512b30d9c1e0364e6a7b)