GCC: Wmaybe-uninitialized in indexing_suite_detail

Recent GCC versions (tested on `g++ (GCC) 15.2.1 20260123 (Red Hat 15.2.1-7)` on Fedora 43 with Boost 1.83, but should not differ from newer Boost versions), when compiling this minimal working example:

```
#include
#include
namespace py = boost::python;

BOOST_PYTHON_MODULE(Types) {
py::class_>("StringVector").def(py::vector_indexing_suite>());
}
```

with this command line: `g++ -shared -fPIC Types.cxx -Wextra -O2 -std=c++17 -I/usr/include/python3.14 -I/usr/include/boost -
lboost_python314 -lpython3.14`

throw `Wmaybe-uninitialized`:

```
In file included from /usr/include/c++/15/string:56,
from /usr/include/boost/assert/source_location.hpp:15,
from /usr/include/boost/exception/exception.hpp:9,
from /usr/include/boost/throw_exception.hpp:21,
from /usr/include/boost/function/detail/prologue.hpp:16,
from /usr/include/boost/function/function_template.hpp:13,
from /usr/include/boost/function/detail/maybe_include.hpp:15,
from /usr/include/boost/function/function0.hpp:11,
from /usr/include/boost/python/errors.hpp:13,
from /usr/include/boost/python/handle.hpp:11,
from /usr/include/boost/python/args_fwd.hpp:10,
from /usr/include/boost/python/args.hpp:10,
from /usr/include/boost/python.hpp:11,
from Types.cxx:1:
In member function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size() const [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]’,
inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::length() const [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]’ at /usr/include/c++/15/bits/basic_string.h:1176:20,
inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]’ at /usr/include/c++/15/bits/basic_string.h:617:20,
inlined from ‘void std::__new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string; _Args = {const std::__cxx11::basic_string, std::allocator >&}; _Tp = std::__cxx11::basic_string]’ at /usr/include/c++/15/bits/new_allocator.h:191:4,
inlined from ‘static void std::allocator_traits >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string; _Args = {const std::__cxx11::basic_string, std::allocator >&}; _Tp = std::__cxx11::basic_string]’ at /usr/include/c++/15/bits/alloc_traits.h:674:17,
inlined from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::__cxx11::basic_string; _Alloc = std::allocator >]’ at /usr/include/c++/15/bits/stl_vector.h:1421:30,
inlined from ‘void boost::python::container_utils::extend_container(Container&, boost::python::api::object) [with Container = std::vector >]’ at /usr/include/boost/python/suite/indexing/container_utils.hpp:44:40:
/usr/include/c++/15/bits/basic_string.h:1165:19: warning: ‘*(const std::__cxx11::basic_string, std::allocator >*)((char*)&x + offsetof(boost::python::extract, std::allocator > >,boost::python::extract, std::allocator > >::.boost::python::converter::extract_rvalue, std::allocator > >::m_data.boost::python::converter::rvalue_from_python_data, std::allocator > >::.boost::python::converter::rvalue_from_python_storage, std::allocator > >::storage)).std::__cxx11::basic_string::_M_string_length’ may be used uninitialized [-Wmaybe-uninitialized]
1165 | size_type __sz = _M_string_length;
| ^~~~
```

This can be traced down to originating from this line in `indexing_suite.hpp`: https://github.com/boostorg/python/blob/105d64e8895ab893e5ac9e0380647d216c2cb8d9/include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp#L678

Since this was not touched since Boost 1.83, I believe this to still be relevant. This seems unintentional and due to our project treating warnings as errors, for us this leads to a compile error.