Fix sliced string escaping

[tompng]

Fixes #870

bb = ByteBuffer.wrap(array, offset, length);
bb.getLong(); // Reads from offset and increment offset. ByteBuffer's offset is mutable.
bb.getLong(idx); // Reads from idx, not from offset+idx
bb.getLong(offset + idx); // What we need

ByteBuffer.wrap(array, 0, ptr + len) vs ByteBuffer.wrap(array)

array.length does not match ptr+len for a sliced string. Specifying ptr+len seems more safe.

Test string

'01234567890123456789"a"b"c"d"e"f"g"h'[20, 15]
First 15 bytes don't need escape.
15 = 8 + 4 + 3, which can test getLong, getInt and remaining bytes escaping

[byroot]

Thank you!

[headius]

Aha, another buffer-wrapping issue. Thanks for jumping on this!