Fix ticket reference counting

* Fix kernel panic in case fuse_vget_i fails in fuse_vnop_create

* Fix ticket leak in fuse_vnop_lookup

* Fix ticket leak in fuse_vnop_setxattr caused by not releasing the ticket in
  case fdisp_make_vp_canfail fails

Tracked as issue #84: https://github.com/osxfuse/osxfuse/issues/84
Bump version to 2.5.6