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 ]