Annotate dead code false positive (CID #1504072) (#4646)

Annotate dead code false positive (CID #1504072) (#4646)

Coverity doesn't see that a successful call to fr_base16_decode()
in fr_ldap_util_normalise_dn(), c will be set. Unfortunately, it
appears that each of the ten cases of the switch statement
checking for an escape sequence (other than \\) would then require
annotation.

We therefore use an array of bool const so the check for a valid
escape sequence happens in one line, needing only one annotation.
(Having done this, one might as well use it in fr_ldap_util_is_dn(),
and that is indeed done.)