don't assert on export of ooo74510-1.odt to .docx

the math object is broken in the original .odt so we can't export its contents
as it doesn't have any

Change-Id: I8226cafed65b9e16a4aca6f48a4f711a55532975

sw/source/filter/ww8/docxattributeoutput.cxx

@@ -4392,6 +4392,11 @@ void DocxAttributeOutput::WritePostponedMath(const SwOLENode* pPostponedMath)
 {
     uno::Reference < embed::XEmbeddedObject > xObj(const_cast<SwOLENode*>(pPostponedMath)->GetOLEObj().GetOleRef());
     uno::Reference< uno::XInterface > xInterface( xObj->getComponent(), uno::UNO_QUERY );
+    if (!xInterface.is())
+    {
+        SAL_WARN("sw.ww8", "Broken math object");
+        return;
+    }
 // gcc4.4 (and 4.3 and possibly older) have a problem with dynamic_cast directly to the target class,
 // so help it with an intermediate cast. I'm not sure what exactly the problem is, seems to be unrelated
 // to RTLD_GLOBAL, so most probably a gcc bug.