dirent-util: add several assertions in posix_getdents()

Follow-up for e86a492ff0.

src/basic/dirent-util.h

@@ -34,6 +34,13 @@ struct dirent *readdir_no_dot(DIR *dirp);
  * Let's introduce a simple wrapper. */
 #if !HAVE_POSIX_GETDENTS
 static inline ssize_t posix_getdents(int fd, void *buf, size_t nbyte, int flags) {
+        assert(fd >= 0);
+        assert(buf);
+        assert(nbyte > 0);
+
+        if (flags != 0)
+                return -EINVAL; /* Currently flags must be zero. */
+
         return getdents64(fd, buf, nbyte);
 }
 #endif