gh-159090: Handle deleted StopIteration values in generators

[Shoryamishra61]

Closes gh-159090

Summary

Fixes a crash when a generator or yield from expression receives a StopIteration instance whose value attribute was deleted.

Root cause

When del exc.value is executed on a StopIteration instance, _Py_T_OBJECT member handling clears the C field ((PyStopIterationObject *)exc)->value to NULL. Python attribute lookup exc.value substitutes None, but two internal C consumers dereferenced the field unconditionally:

1. _PyGen_FetchStopIterationValue in Objects/genobject.c called Py_NewRef(NULL) before reaching the if (value == NULL) fallback.
2. CLEANUP_THROW in Python/bytecodes.c called PyStackRef_FromPyObjectNew(NULL).

Fix

• In _PyGen_FetchStopIterationValue (Objects/genobject.c), use Py_XNewRef(...) so that a NULL field returns NULL and triggers the existing fallback if (value == NULL) value = Py_NewRef(Py_None).
• In CLEANUP_THROW (Python/bytecodes.c), check if exc_value->value is non-NULL before creating a stack reference; if NULL, use PyStackRef_None.
• Regenerated bytecode cases via make regen-cases.
• Added user-facing NEWS entry.

Tests

• Added regression tests to Lib/test/test_yield_from.py covering yield from iteration and throw() handling with deleted StopIteration.value, including custom StopIteration subclasses.
• Ran test suites: test_yield_from, test_generators, test_exceptions, and test_coroutines (322 tests passing).
• Ran reference-leak checks on test_yield_from and test_generators with -R 3:3 (0 leaks).
• Ran Tools/patchcheck/patchcheck.py.

Authored with AI assistance and reviewed by human contributor Shoryamishra61.

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