libgui: assign handle to NULL after free

to avoid use-after-free situations

Change-Id: If9c09f509bc55795856302e5ca34470df019c622

libs/ui/GraphicBuffer.cpp

@@ -112,6 +112,7 @@ void GraphicBuffer::free_handle()
         GraphicBufferAllocator& allocator(GraphicBufferAllocator::get());
         allocator.free(handle);
     }
+    handle = NULL;
     mWrappedBuffer = 0;
 }