Fix finally exception chaining on recursion

In this case zend_exception_set_previous() would destroy the
fast_call exception and further accesses on ex would be invalid.
We should on

Fix finally exception chaining on recursion

In this case zend_exception_set_previous() would destroy the
fast_call exception and further accesses on ex would be invalid.
We should only update ex if we update EG(exception).

Fixes oss-fuzz #40464.

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