Fixed behaviour of get_ip_address and get_mac_address for LWIPInterface

per the socket API documentation:

  /** Get the local IP address
   *
   *  @return         Null-terminated representation of the local IP address
   *                  or null if not yet connected
   */
  virtual const char *get_ip_address() = 0;

LWIPInterface incorrectly returned "\0" if unconnected

net/LWIPInterface/LWIPInterface.cpp

@@ -124,12 +124,12 @@ int LWIPInterface::disconnect()
 
 const char *LWIPInterface::get_ip_address()
 {
-    return ip_addr;
+    return ip_addr[0] ? ip_addr : 0;
 }
 
 const char *LWIPInterface::get_mac_address()
 {
-    return mac_addr;
+    return mac_addr[0] ? mac_addr : 0;
 }
 
 struct lwip_socket {