media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()

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

media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()

Wei Chen reports a kernel bug as blew:

general protection fault, probably for non-canonical address

KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]

...

Call Trace:

<TASK>

__i2c_transfer+0x77e/0x1930 drivers/i2c/i2c-core-base.c:2109

i2c_transfer+0x1d5/0x3d0 drivers/i2c/i2c-core-base.c:2170

i2cdev_ioctl_rdwr+0x393/0x660 drivers/i2c/i2c-dev.c:297

i2cdev_ioctl+0x75d/0x9f0 drivers/i2c/i2c-dev.c:458

vfs_ioctl fs/ioctl.c:51 [inline]

__do_sys_ioctl fs/ioctl.c:870 [inline]

__se_sys_ioctl+0xfb/0x170 fs/ioctl.c:856

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

RIP: 0033:0x7fd834a8bded

In az6027_i2c_xfer(), if msg[i].addr is 0x99,

a null-ptr-deref will caused when accessing msg[i].buf.

For msg[i].len is 0 and msg[i].buf is null.

Fix this by checking msg[i].len in az6027_i2c_xfer().