BUG/MINOR: acme: key not restored upon error in acme_res_certificate() V2

BUG/MINOR: acme: key not restored upon error in acme_res_certificate() V2

When receiving the final certificate, it need to be loaded by
ssl_sock_load_pem_into_ckch(). However this function will remove any
existing private key in the struct ckch_store.

In order to fix the issue, the ptr to the key is swapped with a NULL
ptr, and restored once the new certificate is commited.

However there is a discrepancy when there is an error in
ssl_sock_load_pem_into_ckch() fails and the pointer is lost.

This patch fixes the issue by restoring the pointer in the error path.

This must fix issue #2933.