pki: Fix memory leak on error path

Thanks coverity

CID 1445481

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit b90cc79cbe)

src/pki_crypto.c

@@ -833,6 +833,7 @@ ssh_string pki_private_key_to_pem(const ssh_key key,
 
     rc = ssh_string_fill(blob, buf->data, buf->length);
     if (rc < 0) {
+        ssh_string_free(blob);
         goto err;
     }