dhclient(8): Verify lease-, renewal- and rebinding-time option sizes.

Else out-of-bound reads and undefined behaviour may happen.
The current code only checked for the presence of the first of four bytes.
Make sure the fields in question have the minium size required.

No functional change intended.

Reviewed by: rrs@
Sponsored by: NVIDIA Networking

(cherry picked from commit 3492caf512ae090816b4ffa275be43b2f5cfc460)