btrfs: checking NULL or not in some functions

Because NULL is returned when the memory allocation fails,
it is checked whether it is NULL.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index f96641a..9de4ff0 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6496,6 +6496,8 @@
 	int ret = 0;
 
 	ra = kzalloc(sizeof(*ra), GFP_NOFS);
+	if (!ra)
+		return -ENOMEM;
 
 	mutex_lock(&inode->i_mutex);
 	first_index = start >> PAGE_CACHE_SHIFT;