fix issue reading off the end of the ByteBuffer if ptr > 0

[samyron]

Hopefully resolves #859

[byroot]

Do you think you could craft a regression test?

[byroot]

Do you think you could craft a regression test?

Ok, I figured how to repro:

require "json"
s = "01234567890"
JSON.dump(s[2..-1])

[byroot]

I pushed a regression test.

[samyron]

I added a print statement to see what ptr is in that example:

irb(main):001> require "json"
irb(main):002> s = "01234567890"
irb(main):003> JSON.dump(s[2..-1])
ptr: 2

[samyron]

Without looking at the JRuby implementation of String I'm going to guess that s[2..-1] is optimized to change pointers (err, indexes) and not actually rotate the backing array...

[byroot]

Yes, based on https://javadoc.io/doc/org.jruby/jruby-core/9.2.9.0/org/jruby/util/ByteList.html, I assumed JRuby has a bunch of optimization to share the backing byte array between strings. That's how I figured a repro.