LibPDF: Fix security-handler-related nullptr regression

Author: https://github.com/mattco98
Commit: 698fb3957a
Pull-request: https://github.com/SerenityOS/serenity/pull/13326
Reviewed-by: https://github.com/alimpfard

Userland/Applications/PDFViewer/PDFViewerWidget.cpp

@@ -160,7 +160,7 @@ void PDFViewerWidget::open_file(Core::File& file)
 
     auto document = maybe_document.release_value();
 
-    if (auto sh = document->security_handler(); !sh->has_user_password()) {
+    if (auto sh = document->security_handler(); sh && !sh->has_user_password()) {
         // FIXME: Prompt the user for a password
         VERIFY_NOT_REACHED();
     }