Fix libgui cts crash bug

While enabling #define LOG_NDEBUG 0, run camera cts test command:
run cts -m CtsCameraTestCases -t android.hardware.camera2.cts.MultiViewTest#testSharedSurfaceImageReaderSwitch
the libgui will crash due to nullptr, fix this by add nullptr
judgement when pointing to the Graphicbuffer handle
Bug: 228349805

Signed-off-by: tangcheng <tangcheng@xiaomi.com>
Change-Id: I69a84bdb5208b16df88f5f09f45c1a93ad2afe01
Signed-off-by: Simão Gomes Viana <devel@superboring.dev>