Added argument-check to avoid accessing NULL pointer.
The second argument 'interface_name' of this function can be NULL.
And the argument 'interface_name' is used for the argument of strcmp() without checked.
For example, LWIP::add_dns_server() calls dns_getserver(index - 1, NULL).
- Public headers moved into mbed-os/connectivity/lwipstack/include/lwipstack folder
- Source moved into mbed-os/connectivity/lwipstack/source folder
- Internal lwip and lwip-sys libraries moved to source folder