patchtest-send-results: fix sender parsing

Not all mbox 'from' fields will contain angle brackets, so the
re.findall invocation used for getting a reply_address may fail. Use a
simpler reference to the field to get the sender's email address.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>