resolved rhbz#1101224 do not attempt to obtain names for NULL tabs

This happened when the HTML export via clipboard tried to resolve
conditional formats, where we have a temporary instance of a document
containing only the sheet to be exported.

Change-Id: Ic7498a1cab3eabede74773868287a2cc3edef052

sc/source/core/data/document.cxx

@@ -268,9 +268,14 @@ std::vector<OUString> ScDocument::GetAllTableNames() const
     TableContainer::const_iterator it = maTabs.begin(), itEnd = maTabs.end();
     for (; it != itEnd; ++it)
     {
+        // Positions need to be preserved for ScCompiler and address convention
+        // context, so still push an empty string for NULL tabs.
         OUString aName;
+        if (*it)
+        {
             const ScTable& rTab = **it;
             rTab.GetName(aName);
+        }
         aNames.push_back(aName);
     }