BUG/MEDIUM: resolvers: fix truncated TLD consecutive to the API fix

BUG/MEDIUM: resolvers: fix truncated TLD consecutive to the API fix

A bug was introduced by commit previous bf9498a31 ("MINOR: resolvers:
fix the resolv_str_to_dn_label() API about trailing zero") as the code
is particularly contrived and hard to test. The output writes the last
char at [i+1] so the trailing zero and return value must be at i+1.

This will have to be backported where the patch above is backported
since it was needed for a fix.