Fix qqmlecmascript::sequenceConversionThreads with aggressive GC

During the deserialization of a sequence, make sure that when the collector
runs during the filling of the array, that partially filled array is marked
properly.

Fixed setting of arrayDataLen in other similar places as well.

Change-Id: I19926f733c7c7d1398b11b48c1c37f43e7099ead
Reviewed-by: Lars Knoll <lars.knoll@digia.com>