lua: Don't redefine lua_objlen if already defined.

lua 5.2.2 seems to define this symbol already.

(cherry picked from commit 5ee193433a46d205e181bc51080e15f696bbda1a)
(cherry picked from commit 40e3823155c179bf66e05eebfb33e5ff38693b79)
list-filesystems: Do not segfault if guestfs_btrfs_subvolume_list returns an error (RHBZ#1064008).

If calling guestfs_list_filesystems with a disk image containing a
corrupt btrfs volume, the library would segfault.  There was a missing
check for a NULL return from guestfs_btrfs_subvolume_list.

This adds a check, returning the real error up through the stack and
out of guestfs_list_filesystems.

This is potentially a denial of service if processing disk images from
untrusted sources, but is not exploitable.

Thanks: Jeff Bastian for reporting the bug.
(cherry picked from commit d70ceb4cbea165c960710576efac5a5716055486)
Version 1.22.8.