[Java] implement a better fix for an out of bounds exception

[samyron]

After thinking about #859 and the fix #860 all day, something didn't feel quite right. After doing some debugging tonight, I realized the fix might defeat some of the SWAR optimizations.

When creating the ByteBuffer like so ByteBuffer.wrap(ptrBytes, 0, len), we are looking at a different portion of the ptrBytes array when ptr > 0. While we end up reading the correct chunk with bb.getLong(ptr + pos) or bb.getInt(ptr + pos), we may end up terminating the SWAR-optimized loop early.

Consider:

irb(main):001> require "json"
irb(main):002> s = "01234567890"
irb(main):003> JSON.dump(s[2..-1])
ptr: 2
ptrBytes.length: 15
ptrBytes: [48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 48, 0, 0, 0, 0]
len: 9

The backing array has a length of 15 bytes but the length of the string is 9 bytes.

The ByteBuffer in this case is [48, 49, 50, 51, 52, 53, 54, 55, 56] because we start it at index 0. When checking if ptr + pos + 8 <= len we get 2 + 0 + 8 <= 9 which is false.

We really should be looking at [50, 51, 52, 53, 54, 55, 56, 57, 48] assuming we do not offset the bounds check by ptr, as we did prior to the fix earlier today.

There are two possible fixes:

1. Initialize the ByteBuffer as ByteBuffer.wrap(ptrBytes, ptr, len) and remove the ptr offset in the bounds checks.
2. Initialize the ByteBuffer as ByteBuffer.wrap(ptrBytes, 0, ptrBytes.length) and leave the bounds checks offset by ptr.

CC: @headius

[headius]

Either fix would be fine from a style perspective so it's just a matter of what fits the flow of the code best while un-breaking SWAR. I see you went with the first option and I think that's just fine.