Avoid returning undefined bytes in chkpass_in().

Avoid returning undefined bytes in chkpass_in().

We can't really fix the problem that the result is defined to depend on
random(), so it is still going to fail the "unstable input conversion"
test in parse_type.c.  However, we can at least satify valgrind.  (It
looks like this code used to be valgrind-clean, actually, until somebody
did a careless s/strncpy/strlcpy/g on it.)

In passing, let's just make real sure that chkpass_out doesn't overrun
its output buffer.

No need for backpatch, I think, since this is just to satisfy debugging
tools.

Asif Naeem