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()

telethon/client/dialogs.py

@@ -18,7 +18,7 @@ class _DialogsIter(RequestIter):
             hash=0
         )
 
-        if self.limit == 0:
+        if self.limit <= 0:
             # Special case, get a single dialog and determine count
             dialogs = await self.client(self.request)
             self.total = getattr(dialogs, 'count', len(dialogs.dialogs))

telethon/client/messages.py

@@ -111,7 +111,7 @@ class _MessagesIter(RequestIter):
                 hash=0
             )
 
-        if self.limit == 0:
+        if self.limit <= 0:
             # No messages, but we still need to know the total message count
             result = await self.client(self.request)
             if isinstance(result, types.messages.MessagesNotModified):