Declare tentative return types for ext/spl - part 1 (#7115)

Handle ref return from Iterator::key()

Handle this in the implementation of get_current_key of user_it,
so that the callers may assume that the key is not a reference.

Fixes oss

Handle ref return from Iterator::key()

Handle this in the implementation of get_current_key of user_it,
so that the callers may assume that the key is not a reference.

Fixes oss-fuzz #33018.

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