Prevent double-connect causing double-reads later

Which leads to "RuntimeError: readexactly() called while another
coroutine is already waiting for incoming data" being raised,
and causing everything to break or halt.

telethon/network/mtprotosender.py

@@ -54,6 +54,7 @@ class MTProtoSender:
         self._auth_key_callback = auth_key_callback
         self._update_callback = update_callback
         self._auto_reconnect_callback = auto_reconnect_callback
+        self._connect_lock = asyncio.Lock(loop=loop)
 
         # Whether the user has explicitly connected or disconnected.
         #
@@ -114,13 +115,15 @@ class MTProtoSender:
         """
         Connects to the specified given connection using the given auth key.
         """
-        if self._user_connected:
-            self._log.info('User is already connected!')
-            return
+        async with self._connect_lock:
+            if self._user_connected:
+                self._log.info('User is already connected!')
+                return False
 
-        self._connection = connection
-        await self._connect()
-        self._user_connected = True
+            self._connection = connection
+            await self._connect()
+            self._user_connected = True
+            return True
 
     def is_connected(self):
         return self._user_connected