Followup to r1505066, fix 3rd-party FS module loading.

* subversion/libsvn_fs/fs-loader.c
  (get_or_allocate_third): Initialize new struct element.
  (get_library_vtable): Fix loop for 3rd-party modules.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1601411 13f79535-47bb-0310-9956-ffa450edef68