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

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.

Fixes: f8b6285be2964 ("virtio-gpu: add 3d/virgl support")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220327050945.1614-1-xiam0nd.tong@gmail.com
[ kraxel: minor codestyle fixup ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>