[llvm] [flang][runtime] Refine state associated with child I/O (PR #150461)

[via llvm-commits]

ronlieb wrote:

==2250846== Conditional jump or move depends on uninitialised value(s)
==2250846==    at 0x208772: Fortran::runtime::io::IoStatementState& Fortran::runtime::io::ExternalFileUnit::BeginIoStatement<Fortran::runtime::io::ExternalListIoStatementState<(Fortran::runtime::io::Direction)0>, Fortran::runtime::io::ExternalFileUnit&, char const*&, int&>(Fortran::runtime::Terminator const&, Fortran::runtime::io::ExternalFileUnit&, char const*&, int&) (in ./a.out)


https://github.com/llvm/llvm-project/pull/150461