page_load: Disable loading when loading is impossible

When loading the preview fails because we couldn't extract the preview from the
save, be conservative and disable the "Load" button (how could the server load
the file if we can't?). Also hide the preview from the previously selected
item, if any.

See #1166.

client/page_load.cpp

@@ -340,5 +340,14 @@ void page_load::slot_selection_changed(const QItemSelection &selected,
       }
     }
     ui.load_save_text->setText(final_str);
+
+    ui.load_save_text->show();
+    ui.load_pix->show();
+    ui.buttons->button(QDialogButtonBox::Ok)->setEnabled(true);
+  } else {
+    // Couldn't load the save. Clear the preview and prevent loading it.
+    ui.load_save_text->hide();
+    ui.load_pix->hide();
+    ui.buttons->button(QDialogButtonBox::Ok)->setEnabled(false);
   }
 }