lok: don't crash in simple LOK use-case with no callback.

Change-Id: I7bceba10f002ad5751e3d810f9a9767ad2e875bc
Reviewed-on: https://gerrit.libreoffice.org/66924
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

desktop/source/lib/init.cxx

@@ -1577,8 +1577,11 @@ static LibreOfficeKitDocument* lo_documentLoadWithOptions(LibreOfficeKit* pThis,
         }
 
         LibLODocument_Impl* pDocument = new LibLODocument_Impl(xComponent);
-        int nState = doc_getSignatureState(pDocument);
-        pLib->mpCallback(LOK_CALLBACK_SIGNATURE_STATUS, OString::number(nState).getStr(), pLib->mpCallbackData);
+        if (pLib->mpCallback)
+        {
+            int nState = doc_getSignatureState(pDocument);
+            pLib->mpCallback(LOK_CALLBACK_SIGNATURE_STATUS, OString::number(nState).getStr(), pLib->mpCallbackData);
+        }
         return pDocument;
     }
     catch (const uno::Exception& exception)