properly initialized and checked Ethernet init callback (fix #890)

[maidnl]

fix for potential null pointer access / _initializerCallback not initialized #890
This PR properly initializes _initializerCallback function pointer and checks for its validity before to call it

[pennam]

❤️

[manchoz]

@maidnl @pennam see also main...manchoz:ArduinoCore-mbed:rm_initCallback

[pennam]

@maidnl @pennam see also main...manchoz:ArduinoCore-mbed:rm_initCallback

@manchoz are you aware of this callback being used somewhere?

[maidnl]

Thanks @manchoz, however your PR removes completely the callback and this would brake backward compatibility (in case someone used the callback), so I would prefer to keep the callback and just make it safe.
What do you think @pennam ?

[manchoz]

@pennam @maidnl, there is no code referring to this callback. AFAIK, it is a leftover of the early stages of the ArduinoCore-mbed. @facchinm WDYT?

[pennam]

isn't already impossible to use the _initializerCallback() ? eth_if would never be a nullptr because is always initialized here:

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

Line 123 in 0108ded

EthernetInterface *eth_if = &net;

[maidnl]

@pennam, I could be wrong, but here is my understanding:
you have 3 constructors of EthernetClass (1) EthernetClass(EthernetInterface *_if), (2) EthernetClass();, (3) EthernetClass(voidPrtFuncPtr _cb). _initializerCallback() (which is pointer to function) is initialized using constructor (3) and is not related to eth_if. I agree however on the fact that eth_if could not be nullptr ever, but just because you cannot call the constructor EthernetClass(nullptr) because of the conflict between (1) and (3).
This PR is needed in all cases since _initializeCallback is not properly initialized nor there is a check for its validity when is called.

[pennam]

@maidnl i was thinking if it makes any sense to keep the _initializerCallback() in the code:

• using ctor 3 EthernetClass(voidPrtFuncPtr _cb); the _initializerCallback() is never called because eth_if is always different from nullptr
• using ctor 2 EthernetClass(); same story
• using ctor 1 EthernetClass(EthernetInterface *_if) you can teoretically create an Ethernet object with a null eth_if, but you can't configure the _initializerCallback() 😞

[manchoz]

Let's go ahead and remove it. I don't see any use case for this feature.

[pennam]

👍 let's go for #900

[pennam]

superseded by #901