bgpd: Check if FQDN capability length is in valid ranges

bgpd: Check if FQDN capability length is in valid ranges

If FQDN capability comes as dynamic capability we should check if the encoding
is proper.

Before this patch we returned an error if the hostname/domainname length check
was > end. But technically, if the length is also == end, this is
a malformed capability, because we use the data incorrectly after we check the
length.

This causes heap overflow (when compiled with address-sanitizer).

Signed-off-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>