Improve checking of decoded DB2 principal values

Improve checking of decoded DB2 principal values

In krb5_decode_princ_entry(), verify the length of the principal name
before calling krb5_parse_name() or strlen(), to avoid a possible
buffer read overrun.  Check all length fields for negative values.
Avoid performing arithmetic as part of bounds checks.  If the value of
key_data_ver is unexpected, return KRB5_KDB_BAD_VERSION instead of
aborting.

(cherry picked from commit e3d9f03a658e247dbb43cb345aa93a28782fd995)

ticket: 8481
version_fixed: 1.13.7