[OpenMP] Initialize taskgraph-related fields in task allocation

Fix a potential use of uninitialized is_taskgraph and tdg fields when a
task is created outside of a taskgraph construct. These fields were
previously left uninitialized, which could lead to undefined behavior
when later accessed.

Author: jpinot

openmp/runtime/src/kmp_tasking.cpp

@@ -1614,6 +1614,8 @@ kmp_task_t *__kmp_task_alloc(ident_t *loc_ref, kmp_int32 gtid,
   taskdata->td_flags.freed = 0;
 #if OMPX_TASKGRAPH
   taskdata->td_flags.onced = 0;
+  taskdata->is_taskgraph = 0;
+  taskdata->tdg = nullptr;
 #endif
   KMP_ATOMIC_ST_RLX(&taskdata->td_incomplete_child_tasks, 0);
   // start at one because counts current task and children