media: dvb-frontends: dib3000mb: fix uninit-value in dib3000_write_reg

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

media: dvb-frontends: dib3000mb: fix uninit-value in dib3000_write_reg

Syzbot reports [1] an uninitialized value issue found by KMSAN in

dib3000_read_reg().

Local u8 rb[2] is used in i2c_transfer() as a read buffer; in case

that call fails, the buffer may end up with some undefined values.

Since no elaborate error handling is expected in dib3000_write_reg(),

simply zero out rb buffer to mitigate the problem.

[1] Syzkaller report

dvb-usb: bulk message failed: -22 (6/0)

=====================================================

BUG: KMSAN: uninit-value in dib3000mb_attach+0x2d8/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758

dib3000mb_attach+0x2d8/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758

dibusb_dib3000mb_frontend_attach+0x155/0x2f0 drivers/media/usb/dvb-usb/dibusb-mb.c:31

dvb_usb_adapter_frontend_init+0xed/0x9a0 drivers/media/usb/dvb-usb/dvb-usb-dvb.c:290

dvb_usb_adapter_init drivers/media/usb/dvb-usb/dvb-usb-init.c:90 [inline]

dvb_usb_init drivers/media/usb/dvb-usb/dvb-usb-init.c:186 [inline]

dvb_usb_device_init+0x25a8/0x3760 drivers/media/usb/dvb-usb/dvb-usb-init.c:310

dibusb_probe+0x46/0x250 drivers/media/usb/dvb-usb/dibusb-mb.c:110

...

Local variable rb created at:

dib3000_read_reg+0x86/0x4e0 drivers/media/dvb-frontends/dib3000mb.c:54

dib3000mb_attach+0x123/0x3c0 drivers/media/dvb-frontends/dib3000mb.c:758

...