use correct length

python/educational/src/com/jetbrains/edu/EduUtils.java

@@ -94,12 +94,12 @@ public class EduUtils {
         fileWindows = taskDir.createChildData(taskFile, name);
         printWriter = new PrintWriter(new FileOutputStream(fileWindows.getPath()));
         for (AnswerPlaceholder answerPlaceholder : taskFile.getAnswerPlaceholders()) {
-          if (!answerPlaceholder.isValid(document)) {
+          int length = useLength ? answerPlaceholder.getLength() : answerPlaceholder.getPossibleAnswerLength();
+          if (!answerPlaceholder.isValid(document, length)) {
             printWriter.println("#educational_plugin_window = ");
             continue;
           }
           int start = answerPlaceholder.getRealStartOffset(document);
-          int length = useLength ? answerPlaceholder.getLength() : answerPlaceholder.getPossibleAnswerLength();
           final String windowDescription = document.getText(new TextRange(start, start + length));
           printWriter.println("#educational_plugin_window = " + windowDescription);
         }

python/educational/src/com/jetbrains/edu/courseFormat/AnswerPlaceholder.java

@@ -116,6 +116,10 @@ public class AnswerPlaceholder {
   }
 
   public boolean isValid(@NotNull final Document document) {
+    return isValid(document, length);
+  }
+
+  public boolean isValid(@NotNull final Document document, int length) {
     boolean isLineValid = line < document.getLineCount() && line >= 0;
     if (!isLineValid) return false;
     boolean isStartValid = start >= 0 && start < document.getLineEndOffset(line);