[flang] Revert new unintended runtime behavior

[klausler]

Summary

A recent change caused some variable-length sequential formatted
output statements with record positioning at the end of a FORMAT
(e.g., FORMAT('hi',10X) to append blanks at the end of the completed
record when emitting it.

flang/runtime/unit.cpp

if (isFixedRecordLength && recordLength &&
       WriteFrame(
           frameOffsetInFile_, recordOffsetInFrame_ + *recordLength, handler);
       std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord,
           isUnformatted.value_or(false) ? 0 : ' ',
           *recordLength - furthestPositionInRecord);
       furthestPositionInRecord = *recordLength;
     }
     if (!(isFixedRecordLength && access == Access::Direct)) {
+      positionInRecord = furthestPositionInRecord;
       if (isUnformatted.value_or(false)) {
         // Append the length of a sequential unformatted variable-length record
         // as its footer, then overwrite the reserved first four bytes of the
         // record with its length as its header.  These four bytes were skipped
         // over in BeginUnformattedIO<Output>().
         // TODO: Break very large records up into subrecords with negative
         // headers &/or footers
         std::uint32_t length;
         length = furthestPositionInRecord - sizeof length;
-        positionInRecord = furthestPositionInRecord;
         ok = ok &&
             Emit(reinterpret_cast<const char *>(&length), sizeof length,
                 sizeof length, handler);
         positionInRecord = 0;
         ok = ok &&
             Emit(reinterpret_cast<const char *>(&length), sizeof length,
                 sizeof length, handler);
       } else {