dhclient: Fix the trivial buffer overruns correctly

A DHCP client identifier is simply the hardware type (one byte) concatenated
with the hardware address (some variable number of bytes, but at most 16).
Limit the size of the temporary buffer to match and the rest of the
calculations shake out correctly.

This is a follow-up to the incorrect r299512, reverted in r300172.

CIDs: 1008682, 1305550
Sponsored by: EMC / Isilon Storage Division