Fixes the uninitialized-memory exposure in array.array.fromlist() reported in #152166.

fromlist() preallocated n slots with array_resize(self, old_size + n) and then filled them at Py_SIZE(self) - n + i, recomputed from the live Py_SIZE each iteration, while only guarding against the source list changing size. When an element's __index__ resized self as a side effect of the setitem callback, the write index slid forward, the reserved slots were left unwritten, and the array returned uninitialized heap memory (with the real items misplaced) on a successful return — array_resize uses PyMem_RESIZE with no zeroing.

This writes the reserved slot old_size + i directly and raises RuntimeError("array changed size during iteration") if self is resized mid-iteration, mirroring the adjacent list-mutation guard.

Distinct from #144128 / #144138, which fixed a use-after-free in the II/LL/QQ_setitem conversion helpers and did not touch fromlist's index logic; this defect remained on main.

Before

import array
a = array.array('i')
class Evil:
    def __index__(self):
        a.extend([0]*5)
        return 111
a.fromlist([Evil(), 222, 333])
print(a.tolist())
# debug build: [111, -842150451, -842150451, 0, 0, 0, 222, 333]
#                    ^^^^^^^^^^^  ^^^^^^^^^^  0xCDCDCDCD = uninitialized

After

RuntimeError: array changed size during iteration   # array left unchanged

Adds a regression test (test_fromlist_reentrant_self_resize) covering grow and shrink across the signed/unsigned integer typecodes.

• Issue: array.array.fromlist() exposes uninitialized memory when an element's __index__ resizes the array #152166