[OpenMP] Add check for hot_teams array

[jlpeyton]

Summary

If hot teams are not being used, this code could seg fault without the added check, and does so when composability is used in conjunction with nesting. The fix prevents the segfault.

[hbae]

LGTM.

openmp/trunk/runtime/src/kmp_runtime.cpp

if (__kmp_tasking_mode != tskm_immediate_exec) {
           __kmp_free(old_stack);
         }
         // Store master's task_state on stack
         master_th->th
             .th_task_state_memo_stack[master_th->th.th_task_state_top] =
             master_th->th.th_task_state;
         master_th->th.th_task_state_top++;
 #if KMP_NESTED_HOT_TEAMS
-        if (team == master_th->th.th_hot_teams[active_level].hot_team) {
+        if (master_th->th.th_hot_teams &&
+            team == master_th->th.th_hot_teams[active_level].hot_team) {
           // Restore master's nested state if nested hot team
           master_th->th.th_task_state =
               master_th->th
                   .th_task_state_memo_stack[master_th->th.th_task_state_top];
         } else {
 #endif
           master_th->th.th_task_state = 0;
 #if KMP_NESTED_HOT_TEAMS