[PATCH] igmp: check add_grhead() return value

[Alexey Dobriyan <adobriyan@openvz.org>]

OK, now that we aren't seeing crashes which can be attributed to these
NULL dereferences any longer.
--------------------------------------
add_grhead() allocates memory with GFP_ATOMIC and in at least two places skb
from it passed to skb_put() without checking.

Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
---

 net/ipv4/igmp.c  |    2 ++
 net/ipv6/mcast.c |    2 ++
 2 files changed, 4 insertions(+)

--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -455,6 +455,8 @@ static struct sk_buff *add_grec(struct s
 			skb = add_grhead(skb, pmc, type, &pgr);
 			first = 0;
 		}
+		if (!skb)
+			return NULL;
 		psrc = (__be32 *)skb_put(skb, sizeof(__be32));
 		*psrc = psf->sf_inaddr;
 		scount++; stotal++;
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -1582,6 +1582,8 @@ static struct sk_buff *add_grec(struct s
 			skb = add_grhead(skb, pmc, type, &pgr);
 			first = 0;
 		}
+		if (!skb)
+			return NULL;
 		psrc = (struct in6_addr *)skb_put(skb, sizeof(*psrc));
 		*psrc = psf->sf_addr;
 		scount++; stotal++;