media: rtl28xxu: be sure that it won't go past the array size

smatch warns that the RC query code could go past the array size:

	drivers/media/usb/dvb-usb-v2/rtl28xxu.c:1757 rtl2832u_rc_query() error: buffer overflow 'buf' 128 <= 130
	drivers/media/usb/dvb-usb-v2/rtl28xxu.c:1758 rtl2832u_rc_query() error: buffer overflow 'buf' 128 <= 130

The driver logic gets the length of the IR RX buffer with:

        ret = rtl28xxu_rd_reg(d, IR_RX_BC, &buf[0]);
	...
        len = buf[0];

In thesis, this could range between 0 and 255 [1].

While this should never happen in practice, due to hardware limits,
smatch is right when it complains about that, as there's nothing at
the logic that would prevent it. So, if for whatever reason, buf[0]
gets filled by rtl28xx read functions with a value bigger than 128,
it will go past the array.

So, add an explicit check.

[1] I've no idea why smatch thinks that the maximum value is 130.
I double-checked the code several times. Was unable to find any
reason for assuming 130. Perhaps smatch is not properly parsing
u8 here?

Fixes: b5cbaa43a6 ("[media] rtl28xx: initial support for rtl2832u")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/dvb-usb-v2/rtl28xxu.c

@@ -1732,7 +1732,7 @@ static int rtl2832u_rc_query(struct dvb_usb_device *d)
 		goto exit;
 
 	ret = rtl28xxu_rd_reg(d, IR_RX_BC, &buf[0]);
-	if (ret)
+	if (ret || buf[0] > sizeof(buf))
 		goto err;
 
 	len = buf[0];