lib/btree: fix possible NULL pointer dereference

mempool_alloc() can return null in atomic case.
Signed-off-by: Denis Kirjanov <kirjanov@gmail.com>
Cc: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/btree.c

@@ -95,7 +95,8 @@ static unsigned long *btree_node_alloc(struct btree_head *head, gfp_t gfp)
 	unsigned long *node;
 
 	node = mempool_alloc(head->mempool, gfp);
-	memset(node, 0, NODESIZE);
+	if (likely(node))
+		memset(node, 0, NODESIZE);
 	return node;
 }