msgpack: silence uninitialized msg_pack_object warning

Present in the upstream msgpack code. I couldn't find a problematic use,
but this will silence Coverity and is cleaner.

Nothing should access 'via' when the type of a msg_pack_object is NIL.

Coverity report:

1448708 Uninitialized scalar variable

The variable will contain an arbitrary value left from earlier
computations.

In template_callback_root: Use of an uninitialized variable (CWE-457)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4fe63b0279ca7c3f4c34ee8762ec724a731894c5