media: dib7000p: Fix potential division by zero

Variable loopdiv can be assigned 0, then it is used as a denominator,
without checking it for 0.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 713d54a8 ("[media] DiB7090: add support for the dib7090 based")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: (bw != NULL) -> bw]

drivers/media/dvb-frontends/dib7000p.c

@@ -497,7 +497,7 @@ static int dib7000p_update_pll(struct dvb_frontend *fe, struct dibx000_bandwidth
 	prediv = reg_1856 & 0x3f;
 	loopdiv = (reg_1856 >> 6) & 0x3f;
 
-	if ((bw != NULL) && (bw->pll_prediv != prediv || bw->pll_ratio != loopdiv)) {
+	if (loopdiv && bw && (bw->pll_prediv != prediv || bw->pll_ratio != loopdiv)) {
 		dprintk("Updating pll (prediv: old =  %d new = %d ; loopdiv : old = %d new = %d)\n", prediv, bw->pll_prediv, loopdiv, bw->pll_ratio);
 		reg_1856 &= 0xf000;
 		reg_1857 = dib7000p_read_word(state, 1857);