Fix decoding of mkey kvno in mkey_aux tl-data

krb5_dbe_lookup_mkey_aux was decoding a 16-bit value directly into an
int, resulting in the wrong value on big-endian platforms.  The
consequences are mostly invisible because we ignore this field and try
all mkey_aux nodes in krb5_def_fetch_mkey_list.

ticket: 7738 (new)
target_version: 1.12
tags: pullup