[NFC][LSAN] Fix flaky multithreaded test

compiler-rt/test/lsan/TestCases/many_threads_detach.cpp

@@ -5,6 +5,7 @@
 
 #include <pthread.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 // Number of threads to create. This value is greater than kMaxThreads in
 // lsan_thread.cpp so that we can test that thread contexts are not being
@@ -18,7 +19,7 @@ void *null_func(void *args) {
 int main(void) {
   for (size_t i = 0; i < kTestThreads; i++) {
     pthread_t thread;
-    pthread_create(&thread, NULL, null_func, NULL);
+    if (pthread_create(&thread, NULL, null_func, NULL) == 0)
       pthread_detach(thread);
   }
   return 0;