Don't crash on (invalid) hardware address lengths > 127.

These would cause us to crash with a NegativeArraySizeException
when trying to create the clientMac array. Instead, if the length
is > 16 (invalid, because the field is only 16 bytes long), fudge
it to 6 (Ethernet / wifi).  This is a bit less liberal than the
legacy client, which doesn't check the length at all.

Bug: 23725795
Change-Id: I83f47bfc400ffa8ce85dd9d1b8eb96be5afe51a5