Avoid side effects in assert expressions

asserts may be compiled out with -DNDEBUG, so it's wrong to use an
assert expression with an important side effect.

(We also have scores of side-effecting asserts in test programs, but
those are less important and can be dealt with separately.)

(back ported from commit 221cd4a23691601a14500bc00146c265b50bdc94)

https://github.com/krb5/krb5/commit/021eb1fd74dbff614b3760cecde86c7e6ec5ae7f
Author: Tom Yu <tlyu@mit.edu>
Commit: 021eb1fd74dbff614b3760cecde86c7e6ec5ae7f
Branch: krb5-1.9
 src/lib/crypto/krb/cf2.c |    3 ++-
 src/util/et/com_err.c    |   12 ++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)