Handle permission in update of share better

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

apps/files_sharing/lib/Controller/ShareAPIController.php

@@ -808,6 +808,10 @@ class ShareAPIController extends OCSController {
 			throw new OCSNotFoundException($this->l->t('Wrong share ID, share doesn\'t exist'));
 		}
 
+		if ($share->getShareOwner() !== $this->currentUser && $share->getSharedBy() !== $this->currentUser) {
+			throw new OCSForbiddenException('You are not allowed to edit incomming shares');
+		}
+
 		if ($permissions === null &&
 			$password === null &&
 			$sendPasswordByTalk === null &&