Fix leak when the added previous exception is already in the chain

[EdmondDantes]

zend_exception_set_previous() takes ownership of add_previous: every exit either releases it or stores it as previous. When the walk over the exception's previous chain reaches add_previous itself, the loop ends and the function returns without releasing it. Pure PHP reaches it from finally:

function f() {
    $e = new RuntimeException("x");
    try {
        throw $e;
    } finally {
        throw new LogicException("y", 0, $e);
    }
}
try { f(); } catch (Throwable $t) {}

A debug build reports Freeing ... (152 bytes), Total 1 memory leaks detected on PHP-8.4 and master. The fix releases add_previous after the loop, as the other discarding exit does.

Tests, one per caller that reaches the leak from PHP code:

• Zend/tests/try/try_finally_previous_already_in_chain.phpt: an exception thrown in finally (ZEND_HANDLE_EXCEPTION), with the pending exception as the direct previous and one level deeper;
• Zend/tests/exception_previous_already_in_chain_destructor.phpt: a destructor called during unwinding (zend_objects_destroy_object());
• Zend/tests/generators/finally/yield_throw_previous_already_in_chain.phpt: finally of a generator destroyed during unwinding (zend_generator_dtor_storage());
• Zend/tests/fibers/unfinished-fiber-with-throw-previous-already-in-chain.phpt: finally of a fiber destroyed during unwinding (zend_fiber_object_destroy());
• Zend/tests/fibers/gc-destructor-throw-previous-already-in-chain.phpt: GC destructors run in a fiber (remember_prev_exception()).

[Sjord]

Looks good to me.

[ndossche]

Thanks

[LamentXU123]

Thank you.