fix #4214: Two todolist directives break sphinx-1.6.5

CHANGES

@@ -23,6 +23,7 @@ Bugs fixed
 * #4221: napoleon depends on autodoc, but users need to load it manually
 * #2298: automodule fails to document a class attribute
 * #4099: C++: properly link class reference to class from inside constructor
+* #4214: Two todolist directives break sphinx-1.6.5
 
 Testing
 --------

sphinx/ext/todo.py

@@ -178,7 +178,8 @@ def process_todo_nodes(app, doctree, fromdocname):
             todo_entry = todo_info['todo']
             # Remove targetref from the (copied) node to avoid emitting a
             # duplicate label of the original entry when we walk this node.
-            del todo_entry['targetref']
+            if 'targetref' in todo_entry:
+                del todo_entry['targetref']
 
             # (Recursively) resolve references in the todo content
             env.resolve_references(todo_entry, todo_info['docname'],