tdf#115890 sw: fix invalid cast in SwContentTree::GetEntryAltText()

AccessibleListBoxEntry calls this for any entry.

This was always broken but now we have asserts to tell us about it.

Change-Id: I7094a1dfbffd359e0f536ca60cba2478e2a62464
Reviewed-on: https://gerrit.libreoffice.org/57086
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Jenkins
diff --git a/sw/source/uibase/utlui/content.cxx b/sw/source/uibase/utlui/content.cxx
index 9536891..0dabe3c 100644
--- a/sw/source/uibase/utlui/content.cxx
+++ b/sw/source/uibase/utlui/content.cxx
@@ -930,7 +930,7 @@ Size SwContentTree::GetOptimalSize() const

OUString SwContentTree::GetEntryAltText( SvTreeListEntry* pEntry ) const
{
    if( pEntry == nullptr)
    if (pEntry == nullptr || !lcl_IsContent(pEntry))
        return OUString();

    assert(pEntry->GetUserData() == nullptr || dynamic_cast<SwContent*>(static_cast<SwTypeNumber*>(pEntry->GetUserData())));