Handle negative limits gracefully in async generators

We rely on >= 0 for setting the batch size to use (which must
be valid), so it makes sense to make negative limits equal 0.

This is similar to how asyncio.sleep(negative) sleeps 0 seconds,
despite the fact that time.sleep(negative) fails.

telethon/client/chats.py

@@ -37,7 +37,7 @@ class _ParticipantsIter(RequestIter):
                 functions.channels.GetFullChannelRequest(entity)
             )).full_chat.participants_count
 
-            if self.limit == 0:
+            if self.limit <= 0:
                 raise StopAsyncIteration
 
             self.seen = set()