Fix crash if the data is split between CR and LF (cf. irclog commit 68c577bc)

http2irc.py

@@ -495,13 +495,12 @@ class IRCClientProtocol(asyncio.Protocol):
 
 	def data_received(self, data):
 		self.logger.debug(f'Data received: {data!r}')
-		# Split received data on CRLF. If there's any data left in the buffer, prepend it to the first message and process that.
+		# If there's any data left in the buffer, prepend it to the data. Split on CRLF.
 		# Then, process all messages except the last one (since data might not end on a CRLF) and keep the remainder in the buffer.
 		# If data does end with CRLF, all messages will have been processed and the buffer will be empty again.
-		messages = data.split(b'\r\n')
 		if self.buffer:
-			self.message_received(self.buffer + messages[0])
-			messages = messages[1:]
+			data = self.buffer + data
+		messages = data.split(b'\r\n')
 		for message in messages[:-1]:
 			self.message_received(message)
 		self.buffer = messages[-1]