Don't add begin to length for new string slice (#87)

Fixes #86

Author: headius

ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java

@@ -874,7 +874,7 @@ private RubyString newString(Ruby runtime, int start, int length) {
         ByteList byteList = str.getByteList();
         int begin = byteList.begin();
 
-        ByteList newByteList = new ByteList(byteList.unsafeBytes(), begin + start, begin + length, byteList.getEncoding(), true);
+        ByteList newByteList = new ByteList(byteList.unsafeBytes(), begin + start, length, byteList.getEncoding(), true);
 
         return RubyString.newString(runtime, newByteList);
     }

test/strscan/test_stringscanner.rb

@@ -841,4 +841,12 @@ def test_skip_with_begenning_of_line_anchor_not_match
     assert_equal 1, s.skip(/a/)
     assert_nil      s.skip(/^b/)
   end
+
+  # ruby/strscan#86
+  def test_scan_shared_string
+    s = "hellohello"[5..-1]
+    ss = StringScanner.new(s).scan(/hello/)
+
+    assert_equal "hello", ss
+  end
 end