Add null checks to <dirent.h> functions.

Move all the new checks over to the existing __fortify_fatal.

Bug: http://b/67455242
Test: ran tests
Change-Id: Idb899c58c32d52d3b423caf1a91feb7defcba9b3
diff --git a/libc/private/bionic_fortify.h b/libc/private/bionic_fortify.h
index 8591117..7f22963 100644
--- a/libc/private/bionic_fortify.h
+++ b/libc/private/bionic_fortify.h
@@ -26,6 +26,8 @@
  * SUCH DAMAGE.
  */
 
+#pragma once
+
 #include <poll.h> // For struct pollfd.
 #include <stdarg.h>
 #include <stdlib.h>