git-send-email: fix fallback code in extract_valid_address()

git-send-email: fix fallback code in extract_valid_address()

In the fallback check, used when Email::Valid is not available, the
extract_valid_address() uses $1 without checking for success of matching
regex. The $1 variable may still hold the result of previous match,
which is the address when email address was in '<>' or be undefined
otherwise.

Now if match fails undefined value is always returned to indicate error.
The same value is used by Email::Valid->address() in that case.

Previously 'foo@bar' address was rejected by Email::Valid and fallback,
but '<foo@bar>' was rejected by Email::Valid, but accepted by fallback.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>