send-email: don't call methods on undefined values

If SSL verification is enabled in git send-email, we could attempt to call a
method on an undefined value if the verification failed, since $smtp would end
up being undef.  Look up the error string in a way that will produce a helpful
error message and not cause further errors.

Signed-off-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1234,7 +1234,7 @@ X-Mailer: git-send-email $gitversion
 				if ($smtp->code == 220) {
 					$smtp = Net::SMTP::SSL->start_SSL($smtp,
 									  ssl_verify_params())
-						or die "STARTTLS failed! ".$smtp->message;
+						or die "STARTTLS failed! ".IO::Socket::SSL::errstr();
 					$smtp_encryption = '';
 					# Send EHLO again to receive fresh
 					# supported commands