crypto: virtio - Less function calls in __virtio_crypto_akcipher_do_req() after error detection

The kfree() function was called in up to two cases by the
__virtio_crypto_akcipher_do_req() function during error handling
even if the passed variable contained a null pointer.
This issue was detected by using the Coccinelle software.

* Adjust jump targets.

* Delete two initialisations which became unnecessary
  with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>