handle secsi multi-block error

[auosun]

When the secsi message is too long and is split into two blocks, the length of the data in the first block is 244，then When the message is passed in, the header processing sets 'last_block' to false. However, after going through the method shown in the diagram, 'last_block' is overwritten as True, causing incorrect handling of splitting.

I have removed the red portion shown in the diagram in the SecsIMessage class to fix this issue. I'm not sure if it's correct, and I'm unsure if there are any other problems."