media: vidtv: Fix a null-ptr-deref in vidtv_mux_stop_thread

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

media: vidtv: Fix a null-ptr-deref in vidtv_mux_stop_thread

syzbot report a null-ptr-deref in vidtv_mux_stop_thread. [1]

If dvb->mux is not initialized successfully by vidtv_mux_init() in the vidtv_start_streaming(), it will trigger null pointer dereference about mux in vidtv_mux_stop_thread().

Adjust the timing of streaming initialization and check it before stopping it.

[1] KASAN: null-ptr-deref in range [0x0000000000000128-0x000000000000012f] CPU: 0 UID: 0 PID: 5842 Comm: syz-executor248 Not tainted 6.13.0-rc4-syzkaller-00012-g9b2ffa6148b1 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 RIP: 0010:vidtv_mux_stop_thread+0x26/0x80 drivers/media/test-drivers/vidtv/vidtv_mux.c:471 Code: 90 90 90 90 66 0f 1f 00 55 53 48 89 fb e8 82 2e c8 f9 48 8d bb 28 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 02 7e 3b 0f b6 ab 28 01 00 00 31 ff 89 ee e8 RSP: 0018:ffffc90003f2faa8 EFLAGS: 00010202 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff87cfb125 RDX: 0000000000000025 RSI: ffffffff87d120ce RDI: 0000000000000128 RBP: ffff888029b8d220 R08: 0000000000000005 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000003 R12: ffff888029b8d188 R13: ffffffff8f590aa0 R14: ffffc9000581c5c8 R15: ffff888029a17710 FS: 00007f7eef5156c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f7eef5e635c CR3: 0000000076ca6000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace:

vidtv_stop_streaming drivers/media/test-drivers/vidtv/vidtv_bridge.c:209 [inline] vidtv_stop_feed+0x151/0x250 drivers/media/test-drivers/vidtv/vidtv_bridge.c:252 dmx_section_feed_stop_filtering+0x90/0x160 drivers/media/dvb-core/dvb_demux.c:1000 dvb_dmxdev_feed_stop.isra.0+0x1ee/0x270 drivers/media/dvb-core/dmxdev.c:486 dvb_dmxdev_filter_stop+0x22a/0x3a0 drivers/media/dvb-core/dmxdev.c:559 dvb_dmxdev_filter_free drivers/media/dvb-core/dmxdev.c:840 [inline] dvb_demux_release+0x92/0x550 drivers/media/dvb-core/dmxdev.c:1246 __fput+0x3f8/0xb60 fs/file_table.c:450 task_work_run+0x14e/0x250 kernel/task_work.c:239 get_signal+0x1d3/0x2610 kernel/signal.c:2790 arch_do_signal_or_restart+0x90/0x7e0 arch/x86/kernel/signal.c:337 exit_to_user_mode_loop kernel/entry/common.c:111 [inline] exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline] __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline] syscall_exit_to_user_mode+0x150/0x2a0 kernel/entry/common.c:218 do_syscall_64+0xda/0x250 arch/x86/entry/common.c:89 entry_SYSCALL_64_after_hwframe+0x77/0x7f