make krb5_free_* functions ignore NULL

make krb5_free_* functions ignore NULL

This makes them safer to call in various contexts where NULL may leak
through, and makes analysis easier.

Also, remove some checks for NULL before calling a free routine that
will also check for NULL.

ticket: 6377

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21920 dc483132-0cff-0310-8789-dd5450dbe970