[3.13] gh-122712: Guard against __code__ reassignment in CALL_ALLOC_AND_ENTER_INIT (GH-122713)

[mpage]

Verify the necessary properties of the function's code object in CALL_ALLOC_AND_ENTER_INIT to guard against someone reassigning code.

• Issue: Specializing interpreter may crash if the code object of a class's __init__ method is reassigned #122712

[mpage]

JIT failures look like qemu issues and are unrelated to this diff:

ERROR:../../plugins/core.c:221:qemu_plugin_vcpu_init_hook: assertion failed: (success)
Bail out! ERROR:../../plugins/core.c:221:qemu_plugin_vcpu_init_hook: assertion failed: (success)
aarch64-binfmt-P: ./include/qemu/rcu.h:101: rcu_read_unlock: Assertion `p_rcu_reader->depth != 0' failed.

[mpage]

@brandtbucher @markshannon - This is a backport of gh-122713 for 3.13. Happy to continue with this or let you take care of backporting gh-123140 to 3.13.