Remove inappropriate jsonp_free which caused segmentation fault.

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