Fix a mem leak in evp_pkey_copy_downgraded()

Fix a mem leak in evp_pkey_copy_downgraded()

If we get a failure during evp_pkey_copy_downgraded() and on entry *dest
was NULL then we leak the EVP_PKEY that was automatically allocated and
stored in *dest.

Found due to this comment:
https://github.com/openssl/openssl/pull/18355#issuecomment-1145028315

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/18470)