(for 4.9.3) CVE-2018-14882/ICMP6 RPL: Add a missing bounds check

Moreover:
Add and use *_tstr[] strings.
Update four tests outputs accordingly.
Fix a space.

Wang Junjie of 360 ESG Codesafe Team had independently identified this
vulnerability in 2018 by means of fuzzing and provided the packet capture
file for the test.