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.

ChangeLog

@@ -1,3 +1,9 @@
+Changes in version 0.2.1.25 - 2010-??-??
+  o Major bugfixes:
+    - When freeing a cipher, zero it out completely. We only zeroed
+      the first ptrsize bytes. Bugfix on tor-0.0.2pre8. Discovered
+      and patched by ekir. Fixes bug 1254.
+
 Changes in version 0.2.1.24 - 2010-02-21
   Tor 0.2.1.24 makes Tor work again on the latest OS X -- this time
   for sure!

src/common/aes.c

@@ -267,7 +267,7 @@ 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);
 }