related tdf#158914 sc a11y: Release references when disposing sheet

Clear all `ScAccessibleCell` references that are owned by
`ScAccessibleSpreadsheet` when disposing the sheet instead
of when the sheet is deleted.

While `ScAccessibleSpreadsheet::disposing` is always called,
`ScAccessibleSpreadsheet::~ScAccessibleSpreadsheet` is not called
until all of the sheet's `ScAccessibleCell` instances have been
deleted. So make sure that `ScAccessibleSpreadsheet::disposing`
releases all `ScAccessibleCell` references.

This might possibly reduce some of the leaks mentioned in [1].

[1] https://gerrit.libreoffice.org/c/core/+/167961/comments/15a6a4e4_91a77291

Change-Id: Iaf14338945c1899d54c8e7f8a16f38a48316ac98
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167985
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
diff --git a/sc/source/ui/Accessibility/AccessibleSpreadsheet.cxx b/sc/source/ui/Accessibility/AccessibleSpreadsheet.cxx
index fdeea8d..7296863 100644
--- a/sc/source/ui/Accessibility/AccessibleSpreadsheet.cxx
+++ b/sc/source/ui/Accessibility/AccessibleSpreadsheet.cxx
@@ -310,7 +310,11 @@ void SAL_CALL ScAccessibleSpreadsheet::disposing()
        mpViewShell->RemoveAccessibilityObject(*this);
        mpViewShell = nullptr;
    }

    mpAccCell.clear();
    m_mapSelectionSend.clear();
    m_mapFormulaSelectionSend.clear();
    m_pAccFormulaCell.clear();

    ScAccessibleTableBase::disposing();
}