net: ipv4: Fix the warning for dereference

Add a if statements to avoid the warning.

Dan Carpenter report:
The patch faf482ca: "net: ipv4: Move ip_options_fragment() out of
loop" from Aug 23, 2021, leads to the following Smatch complaint:

    net/ipv4/ip_output.c:833 ip_do_fragment()
    warn: variable dereferenced before check 'iter.frag' (see line 828)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: faf482ca ("net: ipv4: Move ip_options_fragment() out of loop")
Link: https://lore.kernel.org/netdev/20210830073802.GR7722@kadam/T/#tSigned-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -825,7 +825,9 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 
 		/* Everything is OK. Generate! */
 		ip_fraglist_init(skb, iph, hlen, &iter);
-		ip_options_fragment(iter.frag);
+
+		if (iter.frag)
+			ip_options_fragment(iter.frag);
 
 		for (;;) {
 			/* Prepare header of the next frame,