[flang] Pad output correctly after tabbing with ADVANCE='no' (bug#63111)

[klausler]

Summary

Correct the code that implements the production of spaces to bring the
furthestPositionInRecord up to a positionInRecord that was tabbed forward
by a T or TR control edit descriptor.

Fixes bug https://github.com/llvm/llvm-project/issues/63111.

flang/runtime/io-stmt.cpp

if constexpr (DIR == Direction::Input) {
     if (mutableModes().nonAdvancing && !InError()) {
       unit().leftTabLimit = unit().furthestPositionInRecord;
     } else {
       FinishReadingRecord();
     }
   } else { // output
     if (mutableModes().nonAdvancing) {
       // Make effects of positioning past the last Emit() visible with blanks.
-      std::int64_t n{unit().positionInRecord - unit().furthestPositionInRecord};
-      while (n-- > 0 && unit().Emit(" ", 1, 1, *this)) {
+      if (unit().positionInRecord > unit().furthestPositionInRecord) {
+        unit().Emit("", 0, 1, *this); // Emit() will pad
       }
       unit().leftTabLimit = unit().positionInRecord;
     } else {
       unit().AdvanceRecord(*this);
     }
     unit().FlushIfTerminal(*this);
   }
   return IoStatementBase::CompleteOperation();