(for 4.9.3) CVE-2018-16228/HNCP: make buffer access safer

print_prefix() has a buffer and does not initialize it. It may call
decode_prefix6(), which also does not initialize the buffer on invalid
input. When that happens, make sure to return from print_prefix() before
trying to print the [still uninitialized] buffer.

This fixes a buffer over-read discovered by Wang Junjie of 360 ESG
Codesafe Team.

Add a test using the capture file supplied by the reporter(s).