Fix error when .proto contains no Messages (e.g. just enums).

Thanks to Paul Fertser for reporting this bug.

generator/nanopb_generator.py

@@ -343,7 +343,7 @@ def toposort2(data):
     '''
     for k, v in data.items():
         v.discard(k) # Ignore self dependencies
-    extra_items_in_deps = reduce(set.union, data.values()) - set(data.keys())
+    extra_items_in_deps = reduce(set.union, data.values(), set()) - set(data.keys())
     data.update(dict([(item, set()) for item in extra_items_in_deps]))
     while True:
         ordered = set(item for item,dep in data.items() if not dep)