fix: Shield against null returned from getContentTypeParameter

Author: ralflang

lib/Mime/Viewer/Plain.php

@@ -90,7 +90,7 @@ protected function _impRender($inline)
         $type = 'text/html; charset=' . $charset;
 
         // Check for 'flowed' text data.
-        if (strcasecmp($this->_mimepart->getContentTypeParameter('format'), 'flowed') == 0) {
+        if (strcasecmp((string)$this->_mimepart->getContentTypeParameter('format'), 'flowed') == 0) {
             $text = $this->_formatFlowed($text, $this->_mimepart->getContentTypeParameter('delsp'));
         } else {
             /* A "From" located at the beginning of a line in the body text