lp#1566050: prevent cyclic reference zombies

regression, likely from 2660d24a:

1/ Since that change mxAccessible in ScCsvGrid holds a rtl::Reference on
a ScAccessibleCsvGrid
2/ Which in turn holds a VclPtr<> (aka a rtl::Reference with lipstick)
on the ScCsvControl

These are a circular references, making both of them live forever and
leak past the point where on LibreOffice close all of Vcl is long gone,
when these are dtored. Clearing mxAccessible on disposing should help.

Change-Id: Iebb2635ec4ea143e7f0dbfebad2e6141a68e72e8
Reviewed-on: https://gerrit.libreoffice.org/24020Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>

sc/source/ui/dbgui/csvcontrol.cxx

@@ -81,6 +81,7 @@ void ScCsvControl::dispose()
 {
     if( mxAccessible.is() )
         mxAccessible->dispose();
+    mxAccessible = nullptr; // lp#1566050: prevent cyclic reference zombies
     Control::dispose();
 }