[PATCH] ISDN: Avoid a potential NULL ptr deref in ippp

There's a potential problem in isdn_ppp.c::isdn_ppp_decompress().
dev_alloc_skb() may fail and return NULL. If it does we will be passing a
NULL skb_out to ipc->decompress() and may also end up
dereferencing a NULL pointer at

    *proto = isdn_ppp_strip_proto(skb_out);

Correct this by testing 'skb_out' against NULL early and bail out.
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/isdn/i4l/isdn_ppp.c

@@ -2536,6 +2536,11 @@ static struct sk_buff *isdn_ppp_decompress(struct sk_buff *skb,struct ippp_struc
   		rsparm.maxdlen = IPPP_RESET_MAXDATABYTES;
   
   		skb_out = dev_alloc_skb(is->mru + PPP_HDRLEN);
+  		if (!skb_out) {
+  			kfree_skb(skb);
+  			printk(KERN_ERR "ippp: decomp memory allocation failure\n");
+			return NULL;
+  		}
 		len = ipc->decompress(stat, skb, skb_out, &rsparm);
 		kfree_skb(skb);
 		if (len <= 0) {