[Truta446]

GH-20403 still reproduces on PHP-8.4 and PHP-8.5: unset($test::$instances[$x]->x) on an uninitialized typed static property reaches the IS_UNDEF container branch of zend_fetch_dimension_address() with a VAR operand, and zval_undefined_cv() reads past the CV names (heap-buffer-overflow under ASan, reproducer from @arnaud-lb in the issue).

As noted in the issue, this is already fixed on master/PHP-8.6 by 8b4ef3a (GH-19160, "Fix FETCH_OBJ_UNSET IS_UNDEF result"), which was only merged into master. This PR backports it to PHP-8.4.

• Commit 1 is a cherry-pick of 8b4ef3a (authorship kept). It needed one adaptation: on PHP-8.4 zend_fetch_static_property_address_ex() returns zend_result, so the new early return is return FAILURE; instead of return NULL;. The two callers that can pass BP_VAR_IS/BP_VAR_UNSET (zend_fetch_static_prop_helper and ZEND_ISSET_ISEMPTY_STATIC_PROP) already treat FAILURE the same way master treats NULL. Zend/zend_vm_execute.h was regenerated with zend_vm_gen.php. NEWS left out.
• Commit 2 adds Zend/tests/gh20403.phpt.

Verification:

• gh20403.phpt fails under --asan on an unpatched PHP-8.5 build and passes with this patch.
• PHP-8.4 debug + ASan: Zend/tests and tests/lang (5281 passed, 0 failed); Zend/tests and ext/opcache/tests with opcache.enable_cli=1, opcache.jit=tracing, opcache.protect_memory=1 (5301 passed, 0 failed).
• PHP-8.4 debug ZTS: Zend/tests (5019 passed, 0 failed).

If you would rather keep this master-only, feel free to close; the test may still be useful there.