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
1 file changed
