[ASAN] fix a nullptr dereference error. (llvm#116011)

`parent_context` is used without checking for nullptr and we can see in
LINE 50 that it could totally be nullptr. This patch addresses this
issue.

compiler-rt/lib/asan/asan_descriptions.cpp

@@ -45,19 +45,17 @@ void DescribeThread(AsanThreadContext *context) {
   }
   context->announced = true;
 
+  InternalScopedString str;
+  str.AppendF("Thread %s", AsanThreadIdAndName(context).c_str());
+
   AsanThreadContext *parent_context =
       context->parent_tid == kInvalidTid
           ? nullptr
           : GetThreadContextByTidLocked(context->parent_tid);
 
   // `context->parent_tid` may point to reused slot. Check `unique_id` which
   // is always smaller for the parent, always greater for a new user.
-  if (context->unique_id <= parent_context->unique_id)
-    parent_context = nullptr;
-
-  InternalScopedString str;
-  str.AppendF("Thread %s", AsanThreadIdAndName(context).c_str());
-  if (!parent_context) {
+  if (!parent_context || context->unique_id <= parent_context->unique_id) {
     str.Append(" created by unknown thread\n");
     Printf("%s", str.data());
     return;