Vulnerability CVE-2018-6948

In CCN-lite 2, the function ccnl_prefix_to_str_detailed can cause a buffer overflow, when writing a prefix to the buffer buf. The maximal size of the prefix is CCNL_MAX_PREFIX_SIZE; the buffer has the size CCNL_MAX_PREFIX_SIZE. However, when NFN is enabled, additional characters are written to the buffer (e.g., the "NFN" and "R2C" tags). Therefore, sending an NFN-R2C packet with a prefix of size CCNL_MAX_PREFIX_SIZE can cause an overflow of buf inside ccnl_prefix_to_str_detailed.