dh: fix libcrypto dh_ctx leak in ssh_dh_cleanup

Ensure to free the `dh_ctx` member in `ssh_dh_cleanup` to match
the allocation in `ssh_dh_init_common`.

The before-and-after of this change can be observed with the pkd
tests and valgrind:

    valgrind --leak-check=full \
      ./pkd_hello -i1 -t torture_pkd_openssh_dsa_rsa_diffie_hellman_group16_sha512

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/dh_crypto.c

@@ -180,6 +180,7 @@ void ssh_dh_cleanup(struct ssh_crypto_struct *crypto)
     if (crypto->dh_ctx != NULL) {
         DH_free(crypto->dh_ctx->keypair[0]);
         DH_free(crypto->dh_ctx->keypair[1]);
+        free(crypto->dh_ctx);
         crypto->dh_ctx = NULL;
     }
 }