Fix 'svnadmin recover' regression crash with old repositories
(--compatible-version=1.3 / 1.2 / 1.1).  Follow-up to r1532029.

* subversion/libsvn_fs_fs/recovery.c
  (svn_fs_fs__find_max_ids): Call svn_fs_fs__open_pack_or_rev_file in order to
    initialize REV_FILE before passing it as an argument to other functions
    (namely, svn_fs_fs__item_offset).

Patch by: Evgeny Kotkov <evgeny.kotkov{_AT_}visualsvn.com>