Dispose unused submenu

Open Writer, Insert > Form Control > Table Control, draw it,
cancel the wizard. Now right click on the table header -> crash.
(can't be tested with gtk/gtk3 vclplug, because GtkSalMenu
holds reference to the menu).

Probably this crashreport:

http://crashreport.libreoffice.org/stats/crash_details/
0bfabafd-3ba4-4c07-8479-399dea07c006

Change-Id: I6588b1e0b5ad4537a4eb98f6a343d0e92264329f

svx/source/fmcomp/fmgridcl.cxx

@@ -728,6 +728,8 @@ void FmGridHeader::PreExecuteColumnContextMenu(sal_uInt16 nColId, PopupMenu& rMe
         pControlMenu->EnableItem(SID_FM_FORMATTEDFIELD + nChangeTypeOffset, bDesignMode && (nColType != TYPE_FORMATTEDFIELD));
         rMenu.SetPopupMenu(SID_FM_CHANGECOL, pControlMenu);
     }
+    else
+        pControlMenu.disposeAndClear();
 
     rMenu.EnableItem(SID_FM_INSERTCOL, bDesignMode && xCols.is());
     rMenu.EnableItem(SID_FM_DELETECOL, bDesignMode && bMarked && xCols.is());