[flang] Add missing call to BeginReadingRecord()

[klausler]

Summary

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.

[Leporacanthicus]

LGTM.

(Sorry, I thought I'd approved it yesterday, but clearly didn't click the submit button!)

flang/runtime/namelist.cpp

   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(