Avoid crash when setting buffer on deleted layer

When setLayerBuffer() was called on a layer previously destroyed by
destroyLayer() the code would crash. Instead, return an error.

Bug: 37159844

Test: Ran vr flinger in a way that would trigger the crash, and
confirmed that I now get error logs instead of crashing. Unfortunately
the error code is consumed by the Composer wrapper class and not
propagated back to the caller, but that's a separate issue (b/37901601).

Change-Id: I75a5b954d47a1deac44d03851f60f347085eca89