krb5_walk_realm_tree leaks in capaths case

krb5_walk_realm_tree leaks in capaths case

Markus Moeller reports a leak in krb5_get_credentials() which was then
traced down to profile strings leaking from within
krb5_walk_realm_tree().  A pointer to a profile string was getting
overwritten without the string being freed when *cap_nodes[0] == '.'.
Fix is to free the string prior to overwriting the pointer if the
pointer is non-null.

ticket: new
target_version: 1.6.2
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19582 dc483132-0cff-0310-8789-dd5450dbe970