udhcpd: fix a bug in add_lease where it was reading at [-1]

It is not correct when we read lease file!

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/serverpacket.c b/networking/udhcp/serverpacket.c
index c3724e0..b48e415 100644
--- a/networking/udhcp/serverpacket.c
+++ b/networking/udhcp/serverpacket.c
@@ -130,7 +130,8 @@
 	uint32_t req_nip;
 	uint32_t lease_time_sec = server_config.max_lease_sec;
 	uint32_t static_lease_ip;
-	uint8_t *req_ip_opt, *p_host_name;
+	uint8_t *req_ip_opt;
+	const char *p_host_name;
 	struct option_set *curr;
 	struct in_addr addr;
 
@@ -173,8 +174,13 @@
 			bb_error_msg("no IP addresses to give - OFFER abandoned");
 			return -1;
 		}
-		p_host_name = get_option(oldpacket, DHCP_HOST_NAME);
-		if (!add_lease(packet.chaddr, packet.yiaddr, server_config.offer_time, p_host_name)) {
+		p_host_name = (const char*) get_option(oldpacket, DHCP_HOST_NAME);
+		if (add_lease(packet.chaddr, packet.yiaddr,
+				server_config.offer_time,
+				p_host_name,
+				p_host_name ? (unsigned char)p_host_name[OPT_LEN - OPT_DATA] : 0
+			) == 0
+		) {
 			bb_error_msg("lease pool is full - OFFER abandoned");
 			return -1;
 		}
@@ -218,7 +224,7 @@
 	struct option_set *curr;
 	uint32_t lease_time_sec;
 	struct in_addr addr;
-	uint8_t *p_host_name;
+	const char *p_host_name;
 
 	init_packet(&packet, oldpacket, DHCPACK);
 	packet.yiaddr = yiaddr;
@@ -242,8 +248,12 @@
 	if (send_packet(&packet, 0) < 0)
 		return -1;
 
-	p_host_name = get_option(oldpacket, DHCP_HOST_NAME);
-	add_lease(packet.chaddr, packet.yiaddr, lease_time_sec, p_host_name);
+	p_host_name = (const char*) get_option(oldpacket, DHCP_HOST_NAME);
+	add_lease(packet.chaddr, packet.yiaddr,
+		lease_time_sec,
+		p_host_name,
+		p_host_name ? (unsigned char)p_host_name[OPT_LEN - OPT_DATA] : 0
+	);
 	if (ENABLE_FEATURE_UDHCPD_WRITE_LEASES_EARLY) {
 		/* rewrite the file with leases at every new acceptance */
 		write_leases();