Fix crasher during multi-thread shaper data creation

Fix crasher during multi-thread shaper data creation

Sample crash:

0  _hb_graphite2_shaper_face_data_destroy (data=0xffffffffffffffff)
    at ../../src/hb-graphite2.cc:129
1  0x00007ffff4271d7d in hb_graphite2_shaper_face_data_ensure (
    face=<optimized out>) at ../../src/hb-shaper-list.hh:35
2  hb_shape_plan_plan (shaper_list=<optimized out>, num_user_features=0,
    user_features=0x0, shape_plan=0xf7b490) at ../../src/hb-shaper-list.hh:35
3  hb_shape_plan_create (face=<optimized out>, props=<optimized out>,
    user_features=0x0, num_user_features=0, shaper_list=<optimized out>)
    at ../../src/hb-shape-plan.cc:108
4  0x00007ffff4272c93 in hb_shape_plan_create_cached (face=0x10cf2b0,
    props=0x11980d8, user_features=0x0, num_user_features=<optimized out>,
    shaper_list=0x0) at ../../src/hb-shape-plan.cc:283