git-send-email: avoid uninitialized variable warning.

The code took length of $reply_to when it was not even defined,
causing -w to warn.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -538,7 +538,7 @@ foreach my $t (@files) {
 	send_message();
 
 	# set up for the next message
-	if ($chain_reply_to || length($reply_to) == 0) {
+	if ($chain_reply_to || !defined $reply_to || length($reply_to) == 0) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= " $message_id";