Resolves: fdo#82747 Crash on opening wikipedia ODT Book

Test if there's indeed a subnode before testing its type

Change-Id: I1bb32652ceb82b0818a8d2cd8d71b46dd96ef2aa
Reviewed-on: https://gerrit.libreoffice.org/11050Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>

starmath/source/mathmlexport.cxx

@@ -866,6 +866,7 @@ void SmXMLExport::ExportTable(const SmNode *pNode, int nLevel)
                 }
                 else if (pTemp->GetType() == NLINE &&
                          pTemp->GetNumSubNodes() == 1 &&
+                         pTemp->GetSubNode(0) &&
                          pTemp->GetSubNode(0)->GetType() == NALIGN)
                 {
                     // For the Table() construction, the NALIGN node is a child