If, in the original code, aRetval[0] was computed before calling
new MaskPrmitive2D (as apparently happens with Clang), no COW-copy of aRetval's
internals was created (as the refcount was still 1), so MaskPrimitive2D's
maChildren would share the Sequence internals with aRetval, so would end up
having itself as its children after the assignment.

Change-Id: I145ca429fdfae2ea555b8c754b407a81b989d45b