kernel: drm/vkms: Fix null-ptr-deref in vkms_release()

In the Linux kernel, the following vulnerability has been resolved: drm/vkms: Fix null-ptr-deref in vkmsrelease A null-ptr-deref is triggered when it tries to destroy the workqueue in vkms-output.composerworkq in vkmsrelease. KASAN: null-ptr-deref in range 0x0000000000000118-0x000000000000011f CP...