libc/mutex: Modify the initialization for mutex attr robust
if CONFIG_PTHREAD_MUTEX_UNSAFE is not enabled, pthread_mutexattrinit doesn't initialize the robust member.
So make initialization for robust to PTHREAD_MUTEX_ROBUST
@@ -119,7 +119,7 @@ int pthread_mutexattr_init(FAR pthread_mutexattr_t *attr)
attr->type = PTHREAD_MUTEX_DEFAULT;
#endif
-#ifdef CONFIG_PTHREAD_MUTEX_BOTH
+#ifndef CONFIG_PTHREAD_MUTEX_UNSAFE
#ifdef CONFIG_PTHREAD_MUTEX_DEFAULT_UNSAFE
attr->robust = PTHREAD_MUTEX_STALLED;
#else
