When a packet was over max_raw_size and zlib failed to compress the
first packet chunk then the final output would be 513 bytes which
exceeded m_max_packet_size of 512.
This occured because the first packet chunk used sublen without
adjusting for the new_length + 1 compression flag added in Compress().
When the packet failed to compress then it was already at its max.
After the first packet, chunk sizes are calculated using max_raw_size
which already accounted for the compress flag. (From #979)
This should fix#2325
c847e3da86