tsg: rpc_send_enqueue_pdu always free buffer

rpc_send_enqueue_pdu returns -1 on error but the type of error isn't
distinguishable. Therefore make sure that the buffer gets always freed.
The only exception to this is when the pdu was already queued. Then the
dequeuing function should take care of freeing the buffer when
processing the pdu.
tsg: fix possible free of initialized memory