pthread_barrier_init: add a test for the count == 0 EINVAL case.

The code was already correct, but we didn't have a test.

Bug: N/A
Test: ran tests
Change-Id: Ie63cbcfbfc2f2df6270aae98f0f4add484d210bc
diff --git a/tests/pthread_test.cpp b/tests/pthread_test.cpp
index af682a9..f2f6e01 100644
--- a/tests/pthread_test.cpp
+++ b/tests/pthread_test.cpp
@@ -2499,6 +2499,11 @@
   }
 }
 
+TEST(pthread, pthread_barrier_init_zero_count) {
+  pthread_barrier_t barrier;
+  ASSERT_EQ(EINVAL, pthread_barrier_init(&barrier, nullptr, 0));
+}
+
 TEST(pthread, pthread_spinlock_smoke) {
   pthread_spinlock_t lock;
   ASSERT_EQ(0, pthread_spin_init(&lock, 0));