Bug fix isolate; use correct offset in error message
Bug fix isolate; use correct offset in error message

When a isolate decoder fails, it includes information about the
offsets of how far in the decoding it got in the error message.
With this change the offset is now correct.

x456/binary,flowbox-public/binary,basvandijk/binary,ezyang/binary,flowbox-public/binary,bergmark/binary,fpco/binary