Fix NPE in deferToPendingTransaction.

Just because a child window has a surface doesn't necessarily
mean the parent window will have a surface. destroySurfaceLocked()
only takes care of setting mAttachedHidden so the child surface
will be invisible, but it may not be destroyed until a later point.

Bug: 30813094
Change-Id: Idb1b03fd61a7537ebfe33bafc93f278c0e6751f4