storage_driver: Don't crash in storagePoolCreateXML

storage_driver: Don't crash in storagePoolCreateXML

In my recent patches I've introduced
virStoragePoolObjIsStarting() which is then used to protect
storage pool definition when the pool object is locked and
unlocked during long running jobs. Well, my patches did not
anticipate that @obj can be NULL under 'cleanup' label in
storagePoolCreateXML() (for instance when parsing XML fails).
This imperfection is causing libvirtd to crash then.

Fixes: 13284a6b83 storage_driver: Protect pool def during startup and build
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>