Fixed crash on deleting custom slide show

Create a custom slide show in Impress, attempt to delete it
-> crash
STL vector::erase returns an iterator pointing to the next element
i.e. not the element we want to remove, evtl. invalid end iterator

Change-Id: Ie3758fc1ae5fc5f2a934cd8bed3de4e84a0b841b

sd/source/ui/dlg/custsdlg.cxx

@@ -165,7 +165,8 @@ IMPL_LINK( SdCustomShowDlg, ClickButtonHdl, void *, p )
         sal_uInt16 nPos = m_pLbCustomShows->GetSelectEntryPos();
         if( nPos != LISTBOX_ENTRY_NOTFOUND )
         {
-            delete *pCustomShowList->erase( pCustomShowList->begin() + nPos );
+            delete (*pCustomShowList)[nPos];
+            pCustomShowList->erase( pCustomShowList->begin() + nPos );
             m_pLbCustomShows->RemoveEntry( nPos );
             m_pLbCustomShows->SelectEntryPos( nPos == 0 ? nPos : nPos - 1 );
             bModified = sal_True;