core: Extending SAN storage caused an NPE for a storage detached from a pool

Extending a SAN storage domain that is detached from a storage pool caused
an NPE due to a bug in the CDA part which caused the CDA evaluation to
continue even though the status validation failed

Change-Id: I4f6687bfe8390142519c2d641badc79fe6a1e0db
Bug-Url: https://bugzilla.redhat.com/1061121
Signed-off-by: Tal Nisan <tnisan@redhat.com>