Fix #102930: crash loading a GP file with hook custom curve

The CurveMapping was just not initialized on file read (now done via
BKE_curvemapping_init).

Pull Request: https://projects.blender.org/blender/blender/pulls/118525

source/blender/blenkernel/intern/gpencil_modifier_legacy.cc

@@ -963,6 +963,7 @@ void BKE_gpencil_modifier_blend_read_data(BlendDataReader *reader, ListBase *lb,
       BLO_read_data_address(reader, &hmd->curfalloff);
       if (hmd->curfalloff) {
         BKE_curvemapping_blend_read(reader, hmd->curfalloff);
+        BKE_curvemapping_init(hmd->curfalloff);
       }
     }
     else if (md->type == eGpencilModifierType_Noise) {