xmlsecurity: avoid crash on empty security context

This should prevent 8cf224c2-f9b9-44e1-9cb0-fc452e570886

Change-Id: I1ad4d8437923eb677d574c2c469cc62378c9c7a6
Reviewed-on: https://gerrit.libreoffice.org/39430Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

xmlsecurity/source/dialogs/certificatechooser.cxx

@@ -153,6 +153,8 @@ void CertificateChooser::ImplInitialize()
 
     for (auto &secContext : mxSecurityContexts)
     {
+        if (!secContext.is())
+            continue;
         auto secEnvironment = secContext->getSecurityEnvironment();
         if (!secEnvironment.is())
             continue;