properties: Fix DHCP segfault

In networks that don't have a DHCP server configured, ipv4 address
allocation fails and the ipv4 structure doesn't get populated. When
the GUI is trying to read the ipv4_config.method field to see whether
it contains "dhcp" string, a segmentation fault is generated.

Ethernet manual configuration behavior remains unchanged after this fix.
diff --git a/properties/ethernet.c b/properties/ethernet.c
index 31db7a0..83b1535 100644
--- a/properties/ethernet.c
+++ b/properties/ethernet.c
@@ -194,7 +194,7 @@
 
 	data->button = button;
 
-	if (g_str_equal(ipv4_config.method, "dhcp") == TRUE)
+	if (!ipv4_config.method || g_str_equal(ipv4_config.method, "dhcp") == TRUE)
 		update_ethernet_ipv4(data, CONNMAN_POLICY_DHCP);
 	else
 		update_ethernet_ipv4(data, CONNMAN_POLICY_MANUAL);
diff --git a/properties/wifi.c b/properties/wifi.c
index bd325ef..69cd6c3 100644
--- a/properties/wifi.c
+++ b/properties/wifi.c
@@ -230,7 +230,7 @@
 
 	data->ipv4_config = ipv4_config;
 
-	if (g_str_equal(ipv4_config.method, "dhcp") == TRUE)
+	if (!ipv4_config.method || g_str_equal(ipv4_config.method, "dhcp") == TRUE)
 		update_wifi_ipv4(data, CONNMAN_POLICY_DHCP);
 	else
 		update_wifi_ipv4(data, CONNMAN_POLICY_MANUAL);