storage: gluster: Don't leak private data when storage file init fails

In a44b7b87 I've introduced a function
that initializes a storage file wrapper object on gluster based volumes.

The initialization function leaks the private data pointer in case of
failure. This patch fixes it.

Reported by John Ferlan.

src/storage/storage_backend_gluster.c

@@ -570,6 +570,7 @@ virStorageFileBackendGlusterInit(virStorageFilePtr file)
 error:
     VIR_FREE(priv->volname);
     glfs_fini(priv->vol);
+    VIR_FREE(priv);
 
     return -1;
 }