Increase safety checks in initialisation code

[Ivan-Velickovic]

While integrating the FATFS code I have found it easy to make hard-to-debug mistakes with the initialisation of cothreads, particularly around either getting stack pointers wrong or passing in the wrong number of stacks for how many cothreads exist.

These issues can be easy to make but quite difficult to debug, it would help to have some more debug checks that validate that the stacks provided are accessible and don't cause faults etc.

[Ivan-Velickovic]

More concrete things to check:

• check for null pointers in stack_ptrs_arg_array_t.
• overflow should be checked first before trying to access the stacks
• alignment of stacks should be checked