ipv4: drop unused assignment

The assignment in the if statement will be overwritten by the
following statement

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -2558,7 +2558,6 @@ int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf,
 	msf->imsf_fmode = pmc->sfmode;
 	psl = rtnl_dereference(pmc->sflist);
 	if (!psl) {
-		len = 0;
 		count = 0;
 	} else {
 		count = psl->sl_count;