Don't overflow the allocated DHCP response buffer.

Don't overflow the allocated DHCP response buffer.

When copying the DHCP/BOOTP response into the allocated buffer, use
the calculated response length instead of the maximum size of a DHCP
packet.  The response length was verified correctly but the very last
step missed to use it.

Found by Caspar Schutijser and malloc.conf(5) "S"

OK deraadt@