rbd: Do not free the secret if it is not set

Not all RBD (Ceph) storage pools have cephx authentication turned on,
so "secret" might not be initialized.

It could also be that the secret couldn't be located.

Only call virSecretFree() if "secret" is initialized earlier.
Signed-off-by: NWido den Hollander <wido@widodh.nl>
(cherry picked from commit d58c8478)

src/storage/storage_backend_rbd.c

@@ -176,7 +176,10 @@ static int virStorageBackendRBDOpenRADOSConn(virStorageBackendRBDStatePtr *ptr,
 cleanup:
     VIR_FREE(secret_value);
     VIR_FREE(rados_key);
-    virSecretFree(secret);
+
+    if (secret != NULL)
+        virSecretFree(secret);
+
     virBufferFreeAndReset(&mon_host);
     VIR_FREE(mon_buff);
     return ret;