Add user network interface check before reading hostname

Author: pennam

connectivity/lwipstack/source/LWIPInterface.cpp

@@ -446,9 +446,11 @@ nsapi_error_t LWIP::add_ethernet_interface(EMAC &emac, bool default_if, OnboardN
     interface->memory_manager = &memory_manager;
     interface->ppp_enabled = false;
 
-    hostname = user_network_interface->get_hostname();
-    if (hostname) {
-        netif_set_hostname(&interface->netif, hostname);
+    if (user_network_interface) {
+        hostname = user_network_interface->get_hostname();
+        if (hostname) {
+            netif_set_hostname(&interface->netif, hostname);
+        }
     }
 
 #if (MBED_MAC_ADDRESS_SUM != MBED_MAC_ADDR_INTERFACE)