net: consider option-length when parsing NIS domain

When parsing option 40 (network information service domain) the
option length is in variable 'oplen' and not in 'size'.

Addresses-Coverity-ID: 492765 Uninitialized variables (UNINIT)
Fixes: 8ab388bfdb ("net: add support to parse the NIS domain for the dhcp options")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

net/bootp.c

@@ -886,7 +886,7 @@ static void dhcp_process_options(uchar *popt, uchar *end)
 		case 40:	/* NIS Domain name */
 			if (net_nis_domain[0] == 0) {
 				size = truncate_sz("NIS Domain Name",
-					sizeof(net_nis_domain), size);
+					sizeof(net_nis_domain), oplen);
 				memcpy(&net_nis_domain, popt + 2, size);
 				net_nis_domain[size] = 0;
 			}