[flang][runtime] Don't crash after surviving internal output overflow

[klausler]

Summary

After the program has survived its attempt to overflow the output buffer
with an internal WRITE using ERR=, IOSTAT=, &/or IOMSG=, don't crash
by accidentally blank-filling the next record that usually doesn't exist.

flang/runtime/internal-unit.cpp

InternalDescriptorUnit<DIR>::InternalDescriptorUnit(
   recordLength = d.ElementBytes();
   endfileRecordNumber = d.Elements() + 1;
 }
 
 template <Direction DIR> void InternalDescriptorUnit<DIR>::EndIoStatement() {
   if constexpr (DIR == Direction::Output) {
     // Clear the remainder of the current record if anything was written
     // to it, or if it is the only record.
-    if (endfileRecordNumber.value_or(-1) == 2 || furthestPositionInRecord > 0) {
+    auto end{endfileRecordNumber.value_or(0)};
+    if (currentRecordNumber < end &&
+        (end == 2 || furthestPositionInRecord > 0)) {
       BlankFillOutputRecord();
     }
   }
 }
 
 template <Direction DIR>
 bool InternalDescriptorUnit<DIR>::Emit(
     const char *data, std::size_t bytes, IoErrorHandler &handler) {