lib/btree: fix possible NULL pointer dereference

mempool_alloc() can return null in atomic case.

Signed-off-by: Denis Kirjanov <[e-mail]>
Cc: Joern Engel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/btree.c

···
 	unsigned long *node;
 
 	node = mempool_alloc(head->mempool, gfp);
-	memset(node, 0, NODESIZE);
+	if (likely(node))
+		memset(node, 0, NODESIZE);
 	return node;
 }