Fix wrong return value check of mmap function

The mmap function never returns NULL. If an error occurs, the function returns MAP_FAILED.

CLA: trivial

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12187)

crypto/mem_sec.c

@@ -514,7 +514,7 @@ static void sh_done(void)
     OPENSSL_free(sh.freelist);
     OPENSSL_free(sh.bittable);
     OPENSSL_free(sh.bitmalloc);
-    if (sh.map_result != NULL && sh.map_size)
+    if (sh.map_result != MAP_FAILED && sh.map_size)
         munmap(sh.map_result, sh.map_size);
     memset(&sh, 0, sizeof(sh));
 }