missing null pointer checks

[ddmesh]

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 6 in dd5abb6

if (eth_if == nullptr) {

Per default an object is created via arduino::EthernetClass Ethernet;. In this case the eth_if member got initialized (which is a pointer).

But when user creates his own object and passes a null pointer for EthernetInterface()
arduino::EthernetClass Ethernet(nullptr);
Several calls to other public functions, will directly call/access the null pointer.

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 17 in dd5abb6

eth_if->get_emac().set_hwaddr(mac);

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 56 in dd5abb6

eth_if->set_dhcp(false);

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 71 in dd5abb6

return (eth_if->get_connection_status() == NSAPI_STATUS_GLOBAL_UP ? LinkON : LinkOFF);

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 80 in dd5abb6

eth_if->disconnect();

[manchoz]

Hey, @ddmesh, would you mind adding the null-pointer checks where needed and submitting a PR?

Thanks!