Fix long-term shutdown/tick fci/fcc storage

Normally incrementing the refcount on just function_name is
sufficient. However, if the callable is of the form 'X::y' inside
an instance

Fix long-term shutdown/tick fci/fcc storage

Normally incrementing the refcount on just function_name is
sufficient. However, if the callable is of the form 'X::y' inside
an instance method, this will capture $this in fcc.object, which
also needs to be retained.

The fci_addref/fci_release helpers should likely be exported as
a general API, as we may have this problem in other places as
well.

Fixes oss-fuzz #39778.

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