Check wp_get_attachment_image_src() return values in _get_image_attachment()

[Whissi]

_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.