btrfs: send: handle path ref underflow in header iterate_inode_ref()

Change BUG_ON to proper error handling if building the path buffer
fails. The pointers are not printed so we don't accidentally leak kernel
addresses.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -1074,7 +1074,15 @@ static int iterate_inode_ref(struct btrfs_root *root, struct btrfs_path *path,
 					ret = PTR_ERR(start);
 					goto out;
 				}
-				BUG_ON(start < p->buf);
+				if (unlikely(start < p->buf)) {
+					btrfs_err(root->fs_info,
+			"send: path ref buffer underflow for key (%llu %u %llu)",
+						  found_key->objectid,
+						  found_key->type,
+						  found_key->offset);
+					ret = -EINVAL;
+					goto out;
+				}
 			}
 			p->start = start;
 		} else {