dtrans: reset DropTarget::m_pDropTarget if it is released

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>

dtrans/source/win32/dnd/target.cxx

@@ -87,6 +87,7 @@ void SAL_CALL DropTarget::disposing()
     {
         CoLockObjectExternal( m_pDropTarget, FALSE, TRUE);
         m_pDropTarget->Release();
+        m_pDropTarget = nullptr;
     }
 
     if( m_oleThreadId)
@@ -171,6 +172,7 @@ void SAL_CALL DropTarget::initialize( const Sequence< Any >& aArguments )
                     // do clean up if drag and drop is not possible
                     CoLockObjectExternal( m_pDropTarget, FALSE, FALSE);
                     m_pDropTarget->Release();
+                    m_pDropTarget = nullptr;
                     m_hWnd= NULL;
                 }
             }
@@ -219,6 +221,7 @@ DWORD WINAPI DndTargetOleSTAFunc(LPVOID pParams)
                         // do clean up if drag and drop is not possible
                         CoLockObjectExternal( pTarget->m_pDropTarget, FALSE, FALSE);
                         pTarget->m_pDropTarget->Release();
+                        pTarget->m_pDropTarget = nullptr;
                         pTarget->m_hWnd= NULL;
                     }
                 }