Make combobox tolerate post-dispose usage better.

Change-Id: I5d7127bfc639591862cea91f0597ef050112b43b
diff --git a/vcl/source/control/combobox.cxx b/vcl/source/control/combobox.cxx
index 94b8f3cfa..4606325 100644
--- a/vcl/source/control/combobox.cxx
+++ b/vcl/source/control/combobox.cxx
@@ -804,6 +804,9 @@ void ComboBox::Modify()

void ComboBox::ImplUpdateFloatSelection()
{
    if (!mpImplLB)
        return;

    // move text in the ListBox into the visible region
    mpImplLB->SetCallSelectionChangedHdl( false );
    if ( !IsMultiSelectionEnabled() )