sanitizer warning

https://github.com/cppalliance/http_proto/actions/runs/20605499846/job/59180338376

Example:
```
./boost/json/detail/string_impl.hpp:338:23: error: 'key.boost::json::string::impl_.boost::json::detail::string_impl::.boost::json::detail::string_impl::::p_.boost::json::detail::string_impl::pointer::t' may be used uninitialized [-Werror=maybe-uninitialized]
338 | char*>(p_.t + 1);
| ~~~^
In file included from ./boost/json/src.hpp:39:
./boost/json/impl/pointer.ipp: In function 'Value* boost::json::detail::walk_pointer(Value&, boost::json::string_view, boost::system::error_code&, OnObject, OnArray, OnScalar) [with Value = boost::json::value; OnObject = boost::json::value::set_at_pointer(boost::json::string_view, boost::json::value_ref, boost::system::error_code&, const boost::json::set_pointer_options&)::; OnArray = boost::json::value::set_at_pointer(boost::json::string_view, boost::json::value_ref, boost::system::error_code&, const boost::json::set_pointer_options&)::; OnScalar = boost::json::value::set_at_pointer(boost::json::string_view, boost::json::value_ref, boost::system::error_code&, const boost::json::set_pointer_options&)::]':
./boost/json/impl/pointer.ipp:438:20: note: 'key' declared here
438 | string key( token.begin(), token.end(), obj.storage() );
| ^~~
```