gh-158928: Fix overflow in bytearray.__init__ from an iterator

[cmaloney]

Passing an iterator to bytearray.__init__ didn't take into account that iterating that iterator could modify the bytearray position and ob_start. When that happened the iterator would write beyond the end of the allocation.

Update the "enough space" check to account for ob_start. The assignment always included it. Add an assert after append that the offsets line up as expected.

• Issue: bytearray.__init__ slow-path append ignores ob_start leading to linear heap OOB write #158928