minor typo in log message

[John Fawcett]

I noticed this minor typo in a log message in util/vstring_vstream.c in
function vstring_get_null_bound

I guess

    if (bound <= 0)
        msg_panic("vstring_get_nonl_bound: invalid bound %ld", (long)
bound);

should be

    if (bound <= 0)
        msg_panic("vstring_get_null_bound: invalid bound %ld", (long)
bound);

John