Don't try to sign when no certificate is selected

Change-Id: I30f8f5970538db25ae678dc5b32afb292fd3983a
Reviewed-on: https://gerrit.libreoffice.org/54652
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

cui/source/dialogs/SignSignatureLineDialog.cxx

@@ -142,6 +142,7 @@ void SignSignatureLineDialog::Apply()
     if (!m_xSelectedCertifate.is())
     {
         SAL_WARN("cui.dialogs", "No certificate selected!");
+        return;
     }
 
     SfxObjectShell* pShell = SfxObjectShell::Current();