Zero a cipher completely before freeing it

We used to only zero the first ptrsize bytes of the cipher. Since
cipher is large enough, we didn't zero too many bytes. Discovered
and fixed by ekir. Fixes bug 1254.

src/common/aes.c

@@ -268,7 +268,7 @@ aes_free_cipher(aes_cnt_cipher_t *cipher)
 #ifdef USE_OPENSSL_EVP
   EVP_CIPHER_CTX_cleanup(&cipher->key);
 #endif
-  memset(cipher, 0, sizeof(cipher));
+  memset(cipher, 0, sizeof(aes_cnt_cipher_t));
   tor_free(cipher);
 }