Fix various integer issues

Fix various integer issues

In kdc_util.c and spnego_mech.c, error returns from ASN.1 length
functions could be ignored because they were assigned to unsigned
values.  In spnego_mech.c, two buffer size checks could be rewritten
to reduce the likelihood of pointer overflow.  In dump.c and
kdc_preauth.c, calloc() could be used to simplify the code and avoid
multiplication overflow.  In pkinit_clnt.c, the wrong value was
checked for a null result from malloc(), and the code could be
simplified.

Reported by Nickolai Zeldovich <nickolai@csail.mit.edu>.

(cherry picked from commit d3c5450ddf0b20855e86dab41735d56c6860156b)

[tlyu@mit.edu: omitted pkinit and kdb5_util fixes because they're not
conservative]

ticket: 7545 (new)
version_fixed: 1.10.4
status: resolved