git-send-email.perl: avoid printing undef when validating addresses

When validating email addresses with `extract_valid_address_or_die()`,
we print out a helpful error message when the given input does not
contain a valid email address.

However, the pre-image of this patch looks something like:

    my $address = shift;
    $address = extract_valid_address($address):
    die sprintf(__("..."), $address) if !$address;

which fails when given a bogus email address by trying to use $address
(which is undef) in a sprintf() expansion, like so:

    $ git.compile send-email --to="pi <pi@pi>" /tmp/x/*.patch --force
    Use of uninitialized value $address in sprintf at /home/ttaylorr/src/git/git-send-email line 1175.
    error: unable to extract a valid address from:

This regression dates back to e431225 (git-send-email: remove invalid
addresses earlier, 2012-11-22), but became more noticeable in a8022c5
(send-email: expose header information to git-send-email's
sendemail-validate hook, 2023-04-19), which validates SMTP headers in
the sendemail-validate hook.

Avoid trying to format an undef by storing the given and cleaned address
separately. After applying this fix, the error contains the invalid
email address, and the warning disappears:

    $ git.compile send-email --to="pi <pi@pi>" /tmp/x/*.patch --force
    error: unable to extract a valid address from: pi <pi@pi>

Reported-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

git-send-email.perl

@@ -1166,10 +1166,10 @@ sub extract_valid_address {
 
 sub extract_valid_address_or_die {
 	my $address = shift;
-	$address = extract_valid_address($address);
+	my $valid_address = extract_valid_address($address);
 	die sprintf(__("error: unable to extract a valid address from: %s\n"), $address)
-		if !$address;
-	return $address;
+		if !$valid_address;
+	return $valid_address;
 }
 
 sub validate_address {