I don't know the details of DHCP, but there is a difference between values supplied by the programmer and those supplied by the user. According to the author, checking if a pointer is not-null is checking a programmer error (EINVAL) so it makes sense to use asserts. But the optval string in the example is supplied by the user so it is insane to use asserts to check for null-termination.

Asserts should only be used to signal "this code is broken" but that is not how they are used here.

A long while ago the GTK+ library used to suffer from a similar affliction. Mainstay GTK+ programs such as GEdit and Totem would print lots of warnings on the console. Since the GTK+ developers used the same facility (g_warning) for reporting both coding errors and for user errors like "file not found," debugging code was hard. Was the warning "supposed to be there" or was it a bug? GTK+ programs are much cleaner these days even if some programs still fails to make the distinction between user and programmer errors.