Objects reuse the GC_PERSISTENT flag as IS_OBJ_WEAKLY_REFERENCED,
which we did not account for in ZVAL_COPY_OR_DUP. To make things
worse the incorrect zval_copy_ctor_func() invocation silently did
nothing. To avoid that, add an assertion that it should only be
called with arrays and strings (unlike the normal zval_copy_ctor()
which can be safely called on any zval).
Nikita Popov