[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

bool IONAME(InputNamelist)(Cookie cookie, const NamelistGroup &group) {
   IoStatementState &io{*cookie};
   io.CheckFormattedStmtType<Direction::Input>("InputNamelist");
   ConnectionState &connection{io.GetConnectionState()};
   connection.modes.inNamelist = true;
   IoErrorHandler &handler{io.GetIoErrorHandler()};
   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(
         "NAMELIST input group does not begin with '&' (at '%lc')", *next);
     return false;
   }
   io.HandleRelativePosition(1);
   char name[nameBufferSize];