Btrfs: confirmation of value is added before trace_btrfs_get_extent() is called

We should confirm the value of extent_map before calling
trace_btrfs_get_extent() because the value of extent_map has the
possibility of NULL.
Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>

fs/btrfs/inode.c

@@ -5505,6 +5505,7 @@ struct extent_map *btrfs_get_extent(struct inode *inode, struct page *page,
 	write_unlock(&em_tree->lock);
 out:
 
+	if (em)
 		trace_btrfs_get_extent(root, em);
 
 	if (path)