Fix crash in SpriteSequence

When QML declares sprites and goalSprite in wrong order the goalsprite
attempts to set it on null spriteEngine. This patch ensures that the
order doesn't matter anymore.

Task-number: QTBUG-40595
Change-Id: I57f1c8754b2e2af91e0c7f72d1d67fec3ad4ede5
Reviewed-by: Lars Knoll <lars.knoll@digia.com>