When read_property fails, it may return `&EG(uninitialized_zval)`, and
the exception is handled in the VM. The VM will try to
`zval_ptr_dtor_nogc` the result, but the result was never set, resulting
in dtor'ing garbage data. To solve this, we check when a different zval*
was returned and initialize the result with UNDEF. We don't need to copy
as the slow_ex handler return values are used directly in a register.
Closes GH-17749.
Niels Dossche
34d8befe8d