Fix error check in nvlist_print_json_string

Move check for errors from mbrtowc() into the loop. The error values
are not actually negative, so we don't break out of the loop when they
are encountered.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12175
Closes #12176