media: dvb-frontends: dib7090p: fix null-ptr-deref in dib7090p_rw_on_apb()

In dib7090p_rw_on_apb, msg is controlled by user. When msg[0].buf is null and
msg[0].len is zero, former checks on msg[0].buf would be passed. If accessing
msg[0].buf[2] without sanity check, null pointer deref would happen. We add
check on msg[0].len to prevent crash. Similar issue occurs when access
msg[1].buf[0] and msg[1].buf[1].

Similar commit: commit 0ed554fd769a ("media: dvb-usb: az6027: fix null-ptr-deref in az6027_i2c_xfer()")

Signed-off-by: Alex Guo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-frontends/dib7000p.c

···
 	u16 word;
 
 	if (num == 1) {		/* write */
+		if (msg[0].len < 3)
+			return -EOPNOTSUPP;
 		dib7000p_write_word(state, apb_address, ((msg[0].buf[1] << 8) | (msg[0].buf[2])));
 	} else {
+		if (msg[1].len < 2)
+			return -EOPNOTSUPP;
 		word = dib7000p_read_word(state, apb_address);
 		msg[1].buf[0] = (word >> 8) & 0xff;
 		msg[1].buf[1] = (word) & 0xff;