use memset_s

lib roken includes support for memset_s() but it was not applied
to the Heimdal source tree.

Change-Id: I8362ec97a9be50205bb2d398e65b629b88ce1acd

lib/ntlm/ntlm.c

@@ -1192,7 +1192,7 @@ splitandenc(unsigned char *hash,
     EVP_CipherInit_ex(&ctx, EVP_des_cbc(), NULL, key, NULL, 1);
     EVP_Cipher(&ctx, answer, challenge, 8);
     EVP_CIPHER_CTX_cleanup(&ctx);
-    memset(key, 0, sizeof(key));
+    memset_s(key, sizeof(key), 0, sizeof(key));
 }
 
 /**