fix warning: 'cipher_hd' may be used uninitialized in this function

I don't think it's an actual issue, but the new compiler on the mac buildbots
isn't smart enough to tell that.

Change-Id: I759e1afe9c4011d5612be0d15282076be6f5a331
Reviewed-on: https://code.wireshark.org/review/3355
Reviewed-by: Evan Huus <eapache@gmail.com>