Initialize DragSource::mDragSourceHelper

At least during CppunitTest_basctl_dialogs_test, DragSource::initialize
apparently throws an exception before initializing mDragSourceHelper, so that

    [mDragSourceHelper release];

in ~DragSource may cause a crash.

Change-Id: Ic11b5d7a1dbfd5636d6f25b30b509d0d8a381262
Reviewed-on: https://gerrit.libreoffice.org/65702
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

vcl/osx/DragSource.cxx

@@ -146,6 +146,7 @@ DragSource::DragSource():
   mView(nullptr),
   mpFrame(nullptr),
   mLastMouseEventBeforeStartDrag(nil),
+  mDragSourceHelper(nil),
   m_MouseButton(0)
 {
 }