safeguard the default value of Product.targetName

Make sure that the default value of Product.targetName is a valid file
name by passing Product.name to qbs.rfc1034Identifier().

Task-number: QBS-637
Change-Id: I722d0d2c78471d17a0111497c7e17d71b349ec57
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>