[OpenMP] Fix tasking bug for decreasing hot team nthreads

[jlpeyton]

Summary

The __kmp_execute_tasks_template() function reads the task_team and
current_task from the thread structure. There appears to be a pathological
timing where the number of threads in the hot team decreases and so a
thread is put in the pool via __kmp_free_thread(). It could be the case where
first: a thread reads th_task_team into task_team local variables

and is then interrupted by the OS

second: the master free's the thread and sets the current task and task team to NULL
third: the thread reads current_task as NULL

When this happens, current_task is dereferenced and a segfault occurs.
This patch just checks for current_task to not be NULL as well.

[hbae]

LGTM.

runtime/src/kmp_tasking.cpp

static inline int __kmp_execute_tasks_template(
   kmp_taskdata_t *current_task = thread->th.th_current_task;
   std::atomic<kmp_int32> *unfinished_threads;
   kmp_int32 nthreads, victim_tid = -2, use_own_tasks = 1, new_victim = 0,
                       tid = thread->th.th_info.ds.ds_tid;
 
   KMP_DEBUG_ASSERT(__kmp_tasking_mode != tskm_immediate_exec);
   KMP_DEBUG_ASSERT(thread == __kmp_threads[gtid]);
 
-  if (task_team == NULL)
+  if (task_team == NULL || current_task == NULL)
     return FALSE;
 
   KA_TRACE(15, ("__kmp_execute_tasks_template(enter): T#%d final_spin=%d "
                 "*thread_finished=%d\n",
                 gtid, final_spin, *thread_finished));
 
   thread->th.th_reap_state = KMP_NOT_SAFE_TO_REAP;
   threads_data = (kmp_thread_data_t *)TCR_PTR(task_team->tt.tt_threads_data);