kernel: virtio-gpu: fix a missing check to avoid NULL dereference

In the Linux kernel, the following vulnerability has been resolved: virtio-gpu: fix a missing check to avoid NULL dereference 'cacheent' could be set NULL inside virtiogpucmdgetcapset and it will lead to a NULL dereference by a lately use of it i.e., ptr = cacheent-capscache. Fix it with a NULL...