NAMELIST input needs to start with a call to BeginReadingRecord().
Internal unit input doesn't care (so unit tests were passing), but
external unit input does need the call and will assert without it.
Differential Revision: https://reviews.llvm.org/D108051
auto *listInput{io.get_if<ListDirectedStatementState<Direction::Input>>()};
RUNTIME_CHECK(handler, listInput != nullptr);
// Check the group header
+ io.BeginReadingRecord();
std::optional<char32_t> next{io.GetNextNonBlank()};
if (!next || *next != '&') {
handler.SignalError(