Bluetooth: btusb: Add date->evt_skb is NULL check

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

Bluetooth: btusb: Add date->evt_skb is NULL check

fix crash because of null pointers

[ 6104.969662] BUG: kernel NULL pointer dereference, address: 00000000000000c8
[ 6104.969667] #PF: supervisor read access in kernel mode
[ 6104.969668] #PF: error_code(0x0000) - not-present page
[ 6104.969670] PGD 0 P4D 0
[ 6104.969673] Oops: 0000 [#1] SMP NOPTI
[ 6104.969684] RIP: 0010:btusb_mtk_hci_wmt_sync+0x144/0x220 [btusb]
[ 6104.969688] RSP: 0018:ffffb8d681533d48 EFLAGS: 00010246
[ 6104.969689] RAX: 0000000000000000 RBX: ffff8ad560bb2000 RCX: 0000000000000006
[ 6104.969691] RDX: 0000000000000000 RSI: ffffb8d681533d08 RDI: 0000000000000000
[ 6104.969692] RBP: ffffb8d681533d70 R08: 0000000000000001 R09: 0000000000000001
[ 6104.969694] R10: 0000000000000001 R11: 00000000fa83b2da R12: ffff8ad461d1d7c0
[ 6104.969695] R13: 0000000000000000 R14: ffff8ad459618c18 R15: ffffb8d681533d90
[ 6104.969697] FS: 00007f5a1cab9d40(0000) GS:ffff8ad578200000(0000) knlGS:00000
[ 6104.969699] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 6104.969700] CR2: 00000000000000c8 CR3: 000000018620c001 CR4: 0000000000760ef0
[ 6104.969701] PKRU: 55555554
[ 6104.969702] Call Trace:
[ 6104.969708] btusb_mtk_shutdown+0x44/0x80 [btusb]
[ 6104.969732] hci_dev_do_close+0x470/0x5c0 [bluetooth]
[ 6104.969748] hci_rfkill_set_block+0x56/0xa0 [bluetooth]
[ 6104.969753] rfkill_set_block+0x92/0x160
[ 6104.969755] rfkill_fop_write+0x136/0x1e0
[ 6104.969759] __vfs_write+0x18/0x40
[ 6104.969761] vfs_write+0xdf/0x1c0
[ 6104.969763] ksys_write+0xb1/0xe0
[ 6104.969765] __x64_sys_write+0x1a/0x20
[ 6104.969769] do_syscall_64+0x51/0x180
[ 6104.969771] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 6104.969773] RIP: 0033:0x7f5a21f18fef
[ 6104.9] RSP: 002b:00007ffeefe39010 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
[ 6104.969780] RAX: ffffffffffffffda RBX: 000055c10a7560a0 RCX: 00007f5a21f18fef
[ 6104.969781] RDX: 0000000000000008 RSI: 00007ffeefe39060 RDI: 0000000000000012
[ 6104.969782] RBP: 00007ffeefe39060 R08: 0000000000000000 R09: 0000000000000017
[ 6104.969784] R10: 00007ffeefe38d97 R11: 0000000000000293 R12: 0000000000000002
[ 6104.969785] R13: 00007ffeefe39220 R14: 00007ffeefe391a0 R15: 000055c10a72acf0