samx3-due: Fix null result from tryAllocateQueue()

[pythagorasw]

When attempting to run stepperConnectToPin() on Arduino Due (SAM3X), a null result is returned.
In sam_queue.cpp: tryAllocateQueue():
The function can only return a valid result if (fas_queue[i]._step_pin == PIN_UNDEFINED) (i.e. the pin is unallocated)

Since _step_pin is initialised by default as 0 on Due, compared to PIN_UNDEFINED = 255, tryAllocateQueue() returns null

Proposed Fix: Update definition in sam_queue.h of the class StepperQueue to explicitly initialise _step_pin as PIN_UNDEFINED:

uint8_t _step_pin = PIN_UNDEFINED;

Tested as working on Arduino Due hardware using PlatformIO

[gin66]

Thanks for the PR.

Could you please check, if the issue is still present in v1.2.1 ?
The overall tryAllocateQueue()/init() has been reworked to exactly avoid this null pointer crash.

[pythagorasw]

The issue is still present in v1.2.1, at least on my hardware (Genuine Arduino Due, 2x units). Upon finding the issue, I updated to the latest version v1.2.1 but encountered the same issue. Issue was present both using PlatformIO, as well as a fresh install of v1.2.1 through the Ardiuno IDE.
The program was able to continue and run a physical stepper motor when I made this change, so I thought I'd at least share my issue and hotfix, even if it's not the most robust solution here.
Does fas_queue's _step_pin get initialised before tryAllocateQueue() somewhere that I've missed?

[gin66]

Thanks for checking with v1.2.1
Based on your feedback I have revisited the code and your fix is actually solving the issue in the right way. Thanks for your support.