[BUGFIX] Custom Swift_Transport impossible

Because of using a wrong variable for checking,
it is impossible to use a custom Swift_Transport
implementation.

Change-Id: Id1a337581642414ff8007de06e7d2e7df153cd79
Resolves: #36937
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/11062
Reviewed-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel

t3lib/mail/class.t3lib_mail_mailer.php

@@ -141,7 +141,7 @@ class t3lib_mail_Mailer extends Swift_Mailer {
 			default:
 					// Custom mail transport
 				$customTransport = t3lib_div::makeInstance($mailSettings['transport'], $mailSettings);
-				if ($this->transport instanceof Swift_Transport) {
+				if ($customTransport instanceof Swift_Transport) {
 					$this->transport = $customTransport;
 				} else {
 					throw new RuntimeException(