Check for unsupported PD exclusion configuration in dhcpv6_parse_ia

We currently only support PD exclusions that only affect bits 64-95 of
the address, so we require:

    32 <= PD prefix length < exclusion prefix length <= 64

The first inequality was not validated, and this could result in a
buffer overflow when generating the next request message.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>