[flang] runtime error on inappropriate OPEN(UNIT=extant,RECL=n)

[klausler]

Summary

Don't let a program set a fixed RECL= on a connected unit unless
it already had one with the same value.

flang/runtime/io-api.cpp

   }
   if (n <= 0) {
     io.GetIoErrorHandler().SignalError("RECL= must be greater than zero");
-  }
-  if (open->wasExtant() &&
-      open->unit().openRecl.value_or(n) != static_cast<std::int64_t>(n)) {
+    return false;
+  } else if (open->wasExtant() &&
+      open->unit().openRecl.value_or(0) != static_cast<std::int64_t>(n)) {
     open->SignalError("RECL= may not be changed for an open unit");
+    return false;
+  } else {
+    open->unit().openRecl = n;
+    return true;
   }
-  open->unit().openRecl = n;
-  return true;
 }
 
 bool IONAME(SetStatus)(Cookie cookie, const char *keyword, std::size_t length) {