Out-of-bounds read when decompressing UDP header

Contiki-NG is an open-source, cross-platform operating system for Next-Generation IoT devices. The 6LoWPAN implementation in Contiki-NG may cast a UDP header structure at a certain offset in a packet buffer. The code does not check whether the packet buffer is large enough to fit a full UDP header structure from the offset where the casting is made. Hence, it is possible to cause an out-of-bounds read beyond the packet buffer. The problem affects anyone running devices with Contiki-NG versions previous to 4.8, and which may receive 6LoWPAN packets from external parties. The problem has been patched in Contiki-NG version 4.8.