Avoid prompting if there are zero prompts

Under some circumstances under Heimdal, the prompter function can
be called with zero prompts.  This resulted in allocating 0 bytes
of memory and then trying to write to it.  Detect that case and
exit early.
Add another Heimdal valgrind suppression

Still not clean, but if I do a little of this with every release,
it will be clean eventually.