Check if DLD contains non-ascii characters

Valid divelogs.de export might contain non-ascii characters in CDATA
fields as long as these characters are found in iso-8859-1. So we'll
have to test to make sure the content is fully ascii before calling
xmlStringLenDecodeEntities to decode possible character references.

Acked-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>