Only sanitize result content on main result page

The other result tabs (images/maps/videos/news) don't have text content
that needs sanitizing.

Fixes #1080

app/filter.py

@@ -161,6 +161,9 @@ class Filter:
         self.update_styling()
         self.remove_block_tabs()
 
+        # self.main_divs is only populated for the main page of search results
+        # (i.e. not images/news/etc).
+        if self.main_divs:
             for div in self.main_divs:
                 self.sanitize_div(div)