Check wp_get_attachment_image_src() return values in _get_image_attachment()

`_get_image_attachment()` could get called with an invalid `$attachment_id` parameter value (for example, when the `get_scan_image()` method extracts an image, the method will pass the complete match first (not just the ID). We need to make sure that we don't set an invalid *image_args* array in this case.