py/vm: Prevent array bound warning when using -MP_OBJ_ITER_BUF_NSLOTS.

[jepler]

This warning can happen on clang 13.0.1 building mpy-cross:

../py/vm.c:748:25: error: array index -3 refers past the last possible
  element for an array in 64-bit address space containing 64-bit (8-byte)
  elements (max possible 2305843009213693952 elements)
  [-Werror,-Warray-bounds]
                        sp[-MP_OBJ_ITER_BUF_NSLOTS + 1] = MP_OBJ_NULL;
                        ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using pointer access instead of array access works around this warning.

This is a cherry-pick of micropython@bb70874

Closes: #6215

[jepler]

Personally I don't get why the change makes the diagnostic disappear (x[y] and *(x+y) refer to the same thing and are valid at the same times) but probably @dpgeorge knows better than me :) :)

[dhalbert]

I think this is related but not exactly the same as the discussion in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578, about using offsets from fixed addresses, and a compiler thinking it knows better than you about whether you are addressing something out of bounds.

[dhalbert]

✔️

[dpgeorge]

Yeah, I'm not sure why this worked, but it did!

We also had other, similar problems that I could only fix with pragmas: micropython@dc91024