media: lgdt3306a: fix a missing check of return value

If lgdt3306a_read_reg() fails, the read data in "val" is incorrect, thus
shouldn't be further used. The fix inserts a check for the return value
of lgdt3306a_read_reg(). If it fails, goto fail.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/dvb-frontends/lgdt3306a.c

@@ -1685,7 +1685,10 @@ static int lgdt3306a_read_signal_strength(struct dvb_frontend *fe,
 	case QAM_256:
 	case QAM_AUTO:
 		/* need to know actual modulation to set proper SNR baseline */
-		lgdt3306a_read_reg(state, 0x00a6, &val);
+		ret = lgdt3306a_read_reg(state, 0x00a6, &val);
+		if (lg_chkerr(ret))
+			goto fail;
+
 		if(val & 0x04)
 			ref_snr = 2800; /* QAM-256 28dB */
 		else