Return none instead of throwing exception when no data is read

hbmqtt/mqtt/packet.py

@@ -110,6 +110,7 @@ class MQTTFixedHeader:
                         raise MQTTException("Invalid remaining length bytes:%s" % bytes_to_hex_str(length_bytes))
             return value
 
+        try:
             b1 = yield from read_or_raise(reader, 1)
             msg_type = decode_message_type(b1)
             if msg_type is PacketType.RESERVED_0 or msg_type is PacketType.RESERVED_15:
@@ -118,6 +119,8 @@ class MQTTFixedHeader:
 
             remain_length = yield from decode_remaining_length()
             return cls(msg_type, flags, remain_length)
+        except NoDataException:
+            return None
 
     def __repr__(self):
         return type(self).__name__ + '(type={0}, length={1}, flags={2})'.format(self.packet_type, self.remaining_length, hex(self.flags))