Troubleshoot "dangling pointer" warning in iterator_interface.t.cpp

[camio]

This warning shows up in the CI job. It was originally pointed out by #13 (review)

/home/runner/work/iterator_interface/iterator_interface/src/beman/iterator_interface26/iterator_interface.t.cpp:5:
/home/runner/work/iterator_interface/iterator_interface/src/beman/iterator_interface26/iterator_interface.t.cpp: In member function ‘virtual void beman::iterator_interface26::IteratorTest_OperatorArrow_Test::TestBody()’:
/home/runner/work/iterator_interface/iterator_interface/src/beman/iterator_interface26/iterator_interface.t.cpp:117:20: warning: using a dangling pointer to an unnamed temporary [-Wdangling-pointer=]
  117 |     ASSERT_EQ(ai->f(), 3);
      |               ~~~~~^~
In file included from /home/runner/work/iterator_interface/iterator_interface/include/beman/iterator_interface26/iterator_interface.hpp:8,
                 from /home/runner/work/iterator_interface/iterator_interface/src/beman/iterator_interface26/iterator_interface.t.cpp:2:
/home/runner/work/iterator_interface/iterator_interface/include/beman/iterator_interface26/detail/stl_interfaces/iterator_interface.hpp:559:59: note: unnamed temporary defined here
  559 |           return detail::make_pointer<pointer, reference>(*derived());
      |                                                           ^~~~~~~~~~