Added argument-check to dns_get_interface_server.
Added argument-check to dns_get_interface_server.

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).

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed