Fix/enable UDP packet reassembly

[szekelyisz]

UdpContext didn't care about pbuf chaining when receiving datagrams, leading
to fragments delivered to the application as individual packets.

[d-a-v]

Thanks for fixing that (probably pending from the introduction of fragment/reassemble lwIP-v2 option).
You took the safe pbuf-logic way for lwip-1.4 (even if I think it does not have this option).

Out of curiosity, what was the UDP protocol which highlighted this bug ?

[szekelyisz]

Thanks @d-a-v! Yeah I scratched my head a little because I found out in the meantime that reassembly is completely disabled in the LwIP 1.4 binary (although the feature is present in the source). Still I chose this approach for two reasons. First, having #ifs around every reference of pbuf->tot_len or pbuf->len is ugly. Second, someone could choose to recompile LwIP 1.4 with reassembly support, and it would be impolite to leave them with the original bug intentionally.

I came across this bug when I wanted to send data for WS2812 LED strips encoded in OSC packets as blobs. The usual MTU of 1500 bytes would leave me with around maximum 470 LEDs per strip, so I was wondering if I could use IP-level fragmentation to squeeze more data into a single datagram and avoid application-level fragmentation/reassembly. After making sure that LwIP had this feature and worked correctly I realized the bug was in the Arduino code. Now I can receive data for thousands of LEDs.