qdoc: ASSERT failure while writing index file

An attempt was made to write an XML attribute after the
atart tag had been closed. This has been fixed by moving
some code around.

Change-Id: I279d91c345f4217dab8fc47faabe3350258d9ece
Task-number: Qt QTBUG-40039
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>