Fix use-after-free in exception trace with user objects

[realFlowControl]

See #371

When a thread throws an exception whose trace contains a plain user object (a class without create_object), the exception is persistently copied via php_parallel_exceptions_save(). The persistent copy retains the thread-local zend_class_entry pointer in dest->ce.

The exception should not hold a pointer to an object in another thread.

[nicolas-grekas]

Makes sense yes, thanks!

FTR, I'm working around the issue by starting my threads with ini_set('zend.exception_ignore_args', '1');