ev-sidebar-thumbnails.c: Don't attempt to generate thumbnails

for epub docs.

Commit 63f21d5 was intended to prevent thumbnails from being
generated for epub docs. Unfortunately, the way default metadata
is generated for documents left open another way for this to be
triggered.

Fixes #674.

Author: mtwebster

Diff for: shell/ev-sidebar-thumbnails.c

@@ -705,6 +705,10 @@ ev_sidebar_thumbnails_fill_model (EvSidebarThumbnails *sidebar_thumbnails)
 	gint prev_width = -1;
 	gint prev_height = -1;
 
+    if (priv->document->iswebdocument) {
+        return;
+    }
+
 	for (i = 0; i < sidebar_thumbnails->priv->n_pages; i++) {
 		gchar     *page_label;
 		gchar     *page_string;