drm/virtio: fix NULL pointer dereference in virtio_gpu_conn_get_modes

In the Linux kernel, the following vulnerability has been resolved:

drm/virtio: fix NULL pointer dereference in virtio_gpu_conn_get_modes

drm_cvt_mode may return NULL and we should check it.

This bug is found by syzkaller:

FAULT_INJECTION stacktrace:
[ 168.567394] FAULT_INJECTION: forcing a failure.
name failslab, interval 1, probability 0, space 0, times 1
[ 168.567403] CPU: 1 PID: 6425 Comm: syz Kdump: loaded Not tainted 4.19.90-vhulk2201.1.0.h1035.kasan.eulerosv2r10.aarch64 #1
[ 168.567406] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[ 168.567408] Call trace:
[ 168.567414] dump_backtrace+0x0/0x310
[ 168.567418] show_stack+0x28/0x38
[ 168.567423] dump_stack+0xec/0x15c
[ 168.567427] should_fail+0x3ac/0x3d0
[ 168.567437] __should_failslab+0xb8/0x120
[ 168.567441] should_failslab+0x28/0xc0
[ 168.567445] kmem_cache_alloc_trace+0x50/0x640
[ 168.567454] drm_mode_create+0x40/0x90
[ 168.567458] drm_cvt_mode+0x48/0xc78
[ 168.567477] virtio_gpu_conn_get_modes+0xa8/0x140 [virtio_gpu]
[ 168.567485] drm_helper_probe_single_connector_modes+0x3a4/0xd80
[ 168.567492] drm_mode_getconnector+0x2e0/0xa70
[ 168.567496] drm_ioctl_kernel+0x11c/0x1d8
[ 168.567514] drm_ioctl+0x558/0x6d0
[ 168.567522] do_vfs_ioctl+0x160/0xf30
[ 168.567525] ksys_ioctl+0x98/0xd8
[ 168.567530] __arm64_sys_ioctl+0x50/0xc8
[ 168.567536] el0_svc_common+0xc8/0x320
[ 168.567540] el0_svc_handler+0xf8/0x160
[ 168.567544] el0_svc+0x10/0x218

KASAN stacktrace:
[ 168.567561] BUG: KASAN: null-ptr-deref in virtio_gpu_conn_get_modes+0xb4/0x140 [virtio_gpu]
[ 168.567565] Read of size 4 at addr 0000000000000054 by task syz/6425
[ 168.567566]
[ 168.567571] CPU: 1 PID: 6425 Comm: syz Kdump: loaded Not tainted 4.19.90-vhulk2201.1.0.h1035.kasan.eulerosv2r10.aarch64 #1
[ 168.567573] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[ 168.567575] Call trace:
[ 168.567578] dump_backtrace+0x0/0x310
[ 168.567582] show_stack+0x28/0x38
[ 168.567586] dump_stack+0xec/0x15c
[ 168.567591] kasan_report+0x244/0x2f0
[ 168.567594] __asan_load4+0x58/0xb0
[ 168.567607] virtio_gpu_conn_get_modes+0xb4/0x140 [virtio_gpu]
[ 168.567612] drm_helper_probe_single_connector_modes+0x3a4/0xd80
[ 168.567617] drm_mode_getconnector+0x2e0/0xa70
[ 168.567621] drm_ioctl_kernel+0x11c/0x1d8
[ 168.567624] drm_ioctl+0x558/0x6d0
[ 168.567628] do_vfs_ioctl+0x160/0xf30
[ 168.567632] ksys_ioctl+0x98/0xd8
[ 168.567636] __arm64_sys_ioctl+0x50/0xc8
[ 168.567641] el0_svc_common+0xc8/0x320
[ 168.567645] el0_svc_handler+0xf8/0x160
[ 168.567649] el0_svc+0x10/0x218