JunitTest_forms_unoapi_3 just crashed in DropTarget::disposing()
with m_pDropTarget pointing to garbage.

Change-Id: Icb5ad15a3d4b857b45553543d404e14abbac4374
(cherry picked from commit 372d4166)
Reviewed-on: https://gerrit.libreoffice.org/17388Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>