Btrfs: do error checking in btrfs_del_csums

Got a report of a box panicing because we got a NULL eb in read_extent_buffer.
His fs was borked and btrfs_search_path returned EIO, but we don't check for
errors so the box paniced. Yes I know this will just make something higher up
the stack panic, but that's a problem for future Josef. Thanks,

Signed-off-by: Josef Bacik <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/file-item.c

···
 			if (path->slots[0] == 0)
 				goto out;
 			path->slots[0]--;
+		} else if (ret < 0) {
+			goto out;
 		}
+
 		leaf = path->nodes[0];
 		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);