tests/optional_mutex: check counter within a lock

The variables "value" and "counter" are modified within mutex locks, so
it makes sense for the final check to also be within a mutex lock.

However, the final check happens after pthread_join(), so there isn't
any danger of synchronization problems.

Reported by:	Coverity scan

tests/optional_mutex/main.c

@@ -95,6 +95,32 @@ test_pthread(void)
 }
 #endif
 
+static int
+check_counter(int counter_expected)
+{
+	int rc = 0;
+
+	/* Lock if we're using pthread. */
+	if ((rc = optional_mutex_lock(&mutex)) != 0) {
+		warn0("optional_mutex_lock: %s", strerror(rc));
+		goto err0;
+	}
+
+	/* Check the value and counter. */
+	if ((value != 0) || (counter != counter_expected))
+		rc = -1;
+
+	/* Unlock if we're using pthread. */
+	if ((rc = optional_mutex_unlock(&mutex)) != 0) {
+		warn0("optional_mutex_unlock: %s", strerror(rc));
+		goto err0;
+	}
+
+err0:
+	/* Success or failure! */
+	return (rc);
+}
+
 int
 main(int argc, char * argv[])
 {
@@ -112,8 +138,8 @@ main(int argc, char * argv[])
 	inc_dec_counter(NULL);
 #endif
 
-	/* Check the counter. */
-	if ((value != 0) || (counter != counter_expected))
+	/* Check that the counter has the expected value. */
+	if (check_counter(counter_expected))
 		goto err0;
 
 	/* Success! */