Investigate untested str4 state in serializer

[grisumbras]

See https://app.codecov.io/gh/boostorg/json/compare/551/tree/include/boost/json/impl/serializer.ipp#D1L220 https://app.codecov.io/gh/boostorg/json/compare/551/tree/include/boost/json/impl/serializer.ipp#D1L339

The state is either untested, because the condition that leads to it is very rare, in which case it still should be recreated in tests. Or this state is genuinely unreachable and should be removed.