Fix use-after-free

Since putOrDelete may delete the layer, we shouldn't be accessing its
member after calling it.

Caught by the static analyzer:

frameworks/base/libs/hwui/renderstate/OffscreenBufferPool.cpp:183:44:
warning: Use of memory after it is freed

Bug: 27101951
Test: mma. Static analyzer warning is gone.
Change-Id: I25ee8b45ede608da52bf58d1d9f52ce3bb60d4cb