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

'cache_ent' could be set NULL inside virtio_gpu_cmd_get_capset()

and it will lead to a NULL dereference by a lately use of it

(i.e., ptr = cache_ent->caps_cache). Fix it with a NULL check.

[ kraxel: minor codestyle fixup ]