[Truffle] Fixed String indexing by offset and length.

ByteList expects the end position and calculates the length itself.  Passing the length ourselves results in a negative index exception.

Author: nirvdrum

core/src/main/java/org/jruby/truffle/nodes/core/StringNodes.java

@@ -242,7 +242,8 @@ public RubyString getIndex(RubyString string, int start, int length) {
             // TODO(CS): why does subSequence return CharSequence?
             final int begin = string.normaliseIndex(start);
             final int exclusiveEnd = string.normaliseIndex(begin + length);
-            return new RubyString(getContext().getCoreLibrary().getStringClass(), (ByteList) string.getBytes().subSequence(begin, exclusiveEnd - begin));
+            
+            return new RubyString(getContext().getCoreLibrary().getStringClass(), (ByteList) string.getBytes().subSequence(begin, exclusiveEnd));
         }
 
     }