pack_string should never free str on error. This wouldn't be a problem
except the check for `ours` was inverted. Just remove the check for
ours since the true condition is unreachable.
json_vpack_ex also had an error check for s.has_error. This can never
be true unless value is NULL.
Expand pack_unpack testing to cover empty format string, NULL object
value with non-null concatenated key, array containing a non-null object
after a NULL (error) string.
Fixes#444
e262ea5fcd