Out-of-bounds read when processing a received IPv6 packet

Contiki-NG is an operating system for internet-of-things devices. In versions 4.9 and prior, when a packet is received, the Contiki-NG network stack attempts to start the periodic TCP timer if it is a TCP packet with the SYN flag set. But the implementation does not first verify that a full TCP header has been received. Specifically, the implementation attempts to access the flags field from the TCP buffer in the following conditional expression in the `check_for_tcp_syn` function. For this reason, an attacker can inject a truncated TCP packet, which will lead to an out-of-bound read from the packet buffer. As of time of publication, a patched version is not available. As a workaround, one can apply the changes in Contiki-NG pull request #2510 to patch the system.