Second consecutive multi-part (TP DT) Memory Access write fails

[sbaconbits]

My test case performs the following operations on a j1939.MemoryAccess
object:

• write len 14
• read len 14
• write len 14
• read len 14

The second write fails to complete.

In the function J1939_21::async_job_thread a message in the state
SENDING_IN_CTS has equal values for 'next_packet_to_send' and 'num_packages'
which I believe should not happen.

Later calls to J1939_21::send_pgn() detects there is a message with the same hash in the
_snd_buffer and returns False.

I believe the fix is to delete the buffer from the send dictionary (_snd_buffer) if
'next_packet_to_send' and 'num_packages' are equal after the call to __send_tp_dt.

With all that being said I am new to using this library and perhaps I'm using
it incorrectly.

Thanks for your time,
Steve

[kellergoech]

the problem is while sending a multipacket message you are not allowed to send another one, that is why made this pull request, this fixed that issue:
#100