Don't crash when post has no media (d'Oh)

mymastotag.py

@@ -27,7 +27,7 @@ def my_masto_tag(lang, instance, user, tag):
         e['description'] = str(soup)
         images = []
         videos =[]
-        for m in e['media_content']:
+        for m in e.get('media_content', []):
             mtype = m['type'].split('/')[0]
             if mtype=='image':
                 images.append(m)