Fix prevent issue - animatableProperty might be null

Fix prevent issue - animatableProperty might be null

- animatableProperty might be null, so move dereferencing code into the null
check block

<Prevent message>
8. var_compare_op: Comparing animatableProperty to null implies that animatableProperty might be null.
1235        if(!animatableProperty)

CID 385005 (#1 of 1): Dereference after null check (FORWARD_NULL)
10. var_deref_op: Dereferencing null pointer animatableProperty.
1245        mAnimatableProperties.PushBack( new AnimatablePropertyMetadata( index, typeInfo->GetComponentIndex(index), animatableProperty->type, animatableProperty->GetSceneGraphProperty() ) );

Change-Id: Ibf6a6670e258b8729b86da80991e0a3ece64452e