Fix UB in tst_QSharedPointer::basics()

Binding a reference to the nullptr is undefined
behavior.

Just skip that particular test when 'ptr' is null.

Found by UBSan:
  tests/auto/corelib/tools/qsharedpointer/tst_qsharedpointer.cpp:258:32: runtime error: reference binding to null pointer of type 'struct Data'

Change-Id: I125588b9d269a6f76716d660d03142f409513885
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>