Fix EZP-23529: Storing a draft with a bad object relation link should not create an entry in ezcontentobject_link

Do not store the ezcontentobject_link entry if the object doesn't exist. (This does not change the behaviour of the draft storing/publishing process.)
Merge pull request #1102 from glye/ezp-23529_bad_object_relation_link

Fix EZP-23529: Storing a draft with a bad object relation link should no...