[OpenMP] Fix a segment fault in __kmp_get_global_thread_id

In `__kmp_get_global_thread_id`, if the gtid mode is 1, after getting the gtid
from TLS, it will store the gtid value to the thread stack maintained in the thread
descriptor. However, `__kmp_get_global_thread_id` can be called when the library
is destructed, after the corresponding thread info has been release. This will
cause a segment fault. This can happen on an Intel-based Mac.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D159324

Author: shiltian

openmp/runtime/src/kmp_runtime.cpp

@@ -196,6 +196,12 @@ int __kmp_get_global_thread_id() {
   if (i < 0)
     return i;
 
+  // other_threads[i] can be nullptr at this point because the corresponding
+  // thread could have already been destructed. It can happen when this function
+  // is called in end library routine.
+  if (!TCR_SYNC_PTR(other_threads[i]))
+    return i;
+
   /* dynamically updated stack window for uber threads to avoid get_specific
      call */
   if (!TCR_4(other_threads[i]->th.th_info.ds.ds_stackgrow)) {