coverity#704999 Unchecked dynamic_cast

Change-Id: I09b2e3f0b1989249fc49ebc3256dd6fcf5762e75
Reviewed-on: https://gerrit.libreoffice.org/8424Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/filter/ww8/ww8atr.cxx

@@ -2869,7 +2869,7 @@ void AttributeOutputBase::TextField( const SwFmtFld& rField )
         {
             const SwInputField * pInputField = dynamic_cast<const SwInputField *>(pFld);
 
-            if (pInputField->isFormField())
+            if (pInputField && pInputField->isFormField())
                 GetExport().DoFormText(pInputField);
             else
             {