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.

ticket: 8481 (new)
target_version: 1.14-next
target_version: 1.13-next