Fix leaks in kadmin server stubs [CVE-2015-8631]

In each kadmind server stub, initialize the client_name and
server_name variables, and release them in the cleanup handler.  Many
of the stubs will otherwise leak the client and server name if
krb5_unparse_name() fails.  Also make sure to free the prime_arg
variables in rename_principal_2_svc(), or we can leak the first one if
unparsing the second one fails.  Discovered by Simo Sorce.

CVE-2015-8631:

In all versions of MIT krb5, an authenticated attacker can cause
kadmind to leak memory by supplying a null principal name in a request
which uses one.  Repeating these requests will eventually cause
kadmind to exhaust all available memory.

    CVSSv2 Vector: AV:N/AC:L/Au:S/C:N/I:N/A:C/E:POC/RL:OF/RC:C

ticket: 8343 (new)
target_version: 1.14-next
target_version: 1.13-next
tags: pullup
static void free_server_handle(kadm5_server_handle_t handle)
{
    if (!handle)
        return;
    krb5_free_principal(handle->context, handle->current_caller);
    free(handle);
}

static void free_server_handle(kadm5_server_handle_t handle)
{
    if (!handle)
        return;
    krb5_free_principal(handle->context, handle->current_caller);
    free(handle);
}
