git-send-email.perl: ensure $domain is defined before using it

valid_fqdn() may attempt to operate on an undefined value if
Net::Domain::domainname fails to determine the domain name.  This causes
perl to emit unpleasant warnings.

So, add a check for whether $domain has been defined before using it.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -895,7 +895,7 @@ sub sanitize_address {
 
 sub valid_fqdn {
 	my $domain = shift;
-	return !($^O eq 'darwin' && $domain =~ /\.local$/) && $domain =~ /\./;
+	return defined $domain && !($^O eq 'darwin' && $domain =~ /\.local$/) && $domain =~ /\./;
 }
 
 sub maildomain_net {