A valid subscriber is indespensible when allocating a new
transaction. Return NULL if no subscriber is supplied. This
will cause unidentified subscribers to be rejected.
Note: Under normal conditions, the problem does not occour,
but it is still possible that a misbehaving MS might trigger
the problem by sending a SETUP command before authenticating
the subscriber. (unencrypted networks)
Change-Id: Ia8739b6e329ab02c0064270d02ad1d6ee245520d
e7cf5e7513