fix memory leaks in notmuch-show.c:format_headers_sprinter()

Internet_address_list_to_string() and
g_mime_message_get_date_as_string() return allocated string buffers
and not const, so from what I can tell from taking a look at the
sprinter-sexp.c’s sexp_string() function, the code leaks the
recipients_string as well as the date string.