Incorrect index for item error path when additionalItems used.

[martinpengellyphillips]

Could be my misunderstanding, but it seems the error path for an item in the below example is being incorrectly set (it should be deque([2]) rather than deque([1])). It only seems to occur when items is non-empty.

from jsonschema import Draft4Validator as Validator

schema = {
    'type': 'array',
    'items': [{
        'type': 'object',
        'properties': {
            'foo': {'type': 'string'}
        },
        'required': ['foo']
    }],
    'additionalItems': {
        'type': 'object',
        'properties': {
            'bar': {'type': 'string'}
        },
        'required': ['bar']
    }
}

instance = [
    {'foo': 'A'},
    {'bar': 'B'},
    {},
]

errors = Validator(schema).iter_errors(instance)
for error in errors:
    print error.path, error.message

# deque([1]) 'bar' is a required property

Note: The aim for this schema is that the first item should be different to all remaining items.

[martinpengellyphillips]

Confirming that proposed pull request #123 fixes the issue (thanks!).