IDEABKL-7547 Decompiler fails to rename class which consists of digits only

src/org/jetbrains/java/decompiler/modules/renamer/ConverterHelper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2015 JetBrains s.r.o.
+ * Copyright 2000-2017 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -55,7 +55,7 @@ public class ConverterHelper implements IIdentifierRenamer {
     }
 
     int index = 0;
-    while (Character.isDigit(shortName.charAt(index))) {
+    while (index < shortName.length() && Character.isDigit(shortName.charAt(index))) {
       index++;
     }