Check substr results

Else this might lead to unexpeted errors.
Found by psalm.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php

@@ -202,7 +202,7 @@ class EmailProvider extends AbstractProvider {
 
 		$organizerEMail = substr($organizer->getValue(), 7);
 
-		if (!$this->mailer->validateMailAddress($organizerEMail)) {
+		if ($organizerEMail === false || !$this->mailer->validateMailAddress($organizerEMail)) {
 			return null;
 		}
 

apps/dav/lib/CalDAV/Schedule/IMipPlugin.php

@@ -178,7 +178,7 @@ class IMipPlugin extends SabreIMipPlugin {
 		// Strip off mailto:
 		$sender = substr($iTipMessage->sender, 7);
 		$recipient = substr($iTipMessage->recipient, 7);
-		if (!$this->mailer->validateMailAddress($recipient)) {
+		if ($recipient === false || !$this->mailer->validateMailAddress($recipient)) {
 			// Nothing to send if the recipient doesn't have a valid email address
 			$iTipMessage->scheduleStatus = '5.0; EMail delivery failed';
 			return;
@@ -239,9 +239,12 @@ class IMipPlugin extends SabreIMipPlugin {
 
 		$message = $this->mailer->createMessage()
 			->setFrom([$fromEMail => $fromName])
-			->setReplyTo([$sender => $senderName])
 			->setTo([$recipient => $recipientName]);
 
+		if ($sender !== false) {
+			$message->setReplyTo([$sender => $senderName]);
+		}
+
 		$template = $this->mailer->createEMailTemplate('dav.calendarInvite.' . $method, $data);
 		$template->addHeader();