Improve length checks in DHCP Options parsing of dhcpcd.

Bug: 26461634
Change-Id: Ic4c2eb381a6819e181afc8ab13891f3fc58b7deb

get_option_uint8(uint8_t *i, const struct dhcp_message *dhcp, uint8_t option)
{
 const uint8_t *p = get_option_raw(dhcp, option);

 if (!p)
 return -1;
 if (i)
 *i = *(p);
 return 0;
}

get_option_uint8(uint8_t *i, const struct dhcp_message *dhcp, uint8_t option)
{
 const uint8_t *p = get_option_raw(dhcp, option);

 if (!p)
 return -1;
 if (i)
 *i = *(p);
 return 0;
}
