As far as I can tell, no DHCPv6 RFC specifies this option. The handling
for the zero option was added in #17736 by @benpicco to fix issues
encountered while trying to retrieve a DHCHPv6 lease. However, I
strongly suspect that the zero option was encountered in this case due
to an out-of-bounds read performed in RIOT's DHCPv6 client
implementation (i.e. the option parsing loop read beyond the packet
bounds). This issue was fixed in #18307 and I strongly suspect that it
should also fix the issue @benpicco originally encountered in #17736. As
such, I propose that we remove the if statement which treats the zero
option as an end-of-payload marker.
Fixes#18309
498e531e2f