Fix an errant memset() into the middle of a struct in cell_pack().

This mistake causes two possible bugs. I believe they are both
harmless IRL.

BUG 1: memory stomping

When we call the memset, we are overwriting two 0 bytes past the end
of packed_cell_t.body. But I think that's harmless in practice,
because the definition of packed_cell_t is:

// ...
typedef struct packed_cell_t {
  TOR_SIMPLEQ_ENTRY(packed_cell_t) next;
  char body[CELL_MAX_NETWORK_SIZE];
  uint32_t inserted_time;
} packed_cell_t;

So we will overwrite either two bytes of inserted_time, or two bytes
of padding, depending on how the platform handles alignment.

If we're overwriting padding, that's safe.

If we are overwriting the inserted_time field, that's also safe: In
every case where we call cell_pack() from connection_or.c, we ignore
the inserted_time field. When we call cell_pack() from relay.c, we
don't set or use inserted_time until right after we have called
cell_pack(). SO I believe we're safe in that case too.

BUG 2: memory exposure

The original reason for this memset was to avoid the possibility of
accidentally leaking uninitialized ram to the network. Now
remember, if wide_circ_ids is false on a connection, we shouldn't
actually be sending more than 512 bytes of packed_cell_t.body, so
these two bytes can only leak to the network if there is another bug
somewhere else in the code that sends more data than is correct.

Fortunately, in relay.c, where we allocate packed_cell_t in
packed_cell_new() , we allocate it with tor_malloc_zero(), which
clears the RAM, right before we call cell_pack. So those
packed_cell_t.body bytes can't leak any information.

That leaves the two calls to cell_pack() in connection_or.c, which
use stack-alocated packed_cell_t instances.

In or_handshake_state_record_cell(), we pass the cell's contents to
crypto_digest_add_bytes(). When we do so, we get the number of
bytes to pass using the same setting of wide_circ_ids as we passed
to cell_pack(). So I believe that's safe.

In connection_or_write_cell_to_buf(), we also use the same setting
of wide_circ_ids in both calls. So I believe that's safe too.

I introduced this bug with 1c0e87f6d8c7a0abdadf1b5cd9082c10abc7f4e2
back in 0.2.4.11-alpha; it is bug 22737 and CID 1401591

changes/bug22737

@@ -0,0 +1,12 @@
+  o Minor bugfixes (defensive programming, undefined behavior):
+
+    - Fix a memset() off the end of an array when packing cells.  This
+      bug should be harmless in practice, since the corrupted bytes
+      are still in the same structure, and are always padding bytes,
+      ignored, or immediately overwritten, depending on compiler
+      behavior. Nevertheless, because the memset()'s purpose is to
+      make sure that any other cell-handling bugs can't expose bytes
+      to the network, we need to fix it. Fixes bug 22737; bugfix on
+      0.2.4.11-alpha. Fixes CID 1401591.
+
+

src/or/connection_or.c

@@ -358,9 +358,11 @@ cell_pack(packed_cell_t *dst, const cell_t *src, int wide_circ_ids)
     set_uint32(dest, htonl(src->circ_id));
     dest += 4;
   } else {
+    /* Clear the last two bytes of dest, in case we can accidentally
+     * send them to the network somehow. */
+    memset(dest+CELL_MAX_NETWORK_SIZE-2, 0, 2);
     set_uint16(dest, htons(src->circ_id));
     dest += 2;
-    memset(dest+CELL_MAX_NETWORK_SIZE-2, 0, 2); /*make sure it's clear */
   }
   set_uint8(dest, src->command);
   memcpy(dest+1, src->payload, CELL_PAYLOAD_SIZE);