Fix double free in EVP_PKEY_CTX_dup()

Fix double free in EVP_PKEY_CTX_dup()

If the internal operations dupctx() fails then a free is done (e.g. EVP_KEYEXCH_free()). If this is not set to NULL the EVP_PKEY_CTX_free() will do a double free.
This was found by testing kdf_dupctx() in kdf_exch.c (Note this always
fails since the internal KDF's do not have a dup method).

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16495)