Fix GH-13998: Manage refcount of agg_context->val correctly (#14004)

When step_callback fails, agg_context->val is passed dtor, but agg_context->val
is also used in final_callback regard

Fix GH-13998: Manage refcount of agg_context->val correctly (#14004)

When step_callback fails, agg_context->val is passed dtor, but agg_context->val
is also used in final_callback regardless of the success/failure of step_callback,
so should not call dtor.

closes #14004
fixes #13998

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