Fix compatibility with PHP 5.2. in rcube_imap_generic (#1490115)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix compatibility with PHP 5.2. in rcube_imap_generic (#1490115)
 - Fix setting flags on servers with no PERMANENTFLAGS response (#1490087)
 - Fix regression in SHAA password generation in ldap driver of password plugin (#1490094)
 - Fix displaying of HTML messages with absolutely positioned elements in Larry skin (#1490103)

program/lib/Roundcube/rcube_imap_generic.php

@@ -865,7 +865,7 @@ class rcube_imap_generic
 
         if (!$this->fp) {
             $this->setError(self::ERROR_BAD, sprintf("Could not connect to %s:%d: %s",
-                $host, $this->prefs['port'], $errstr ?: "Unknown reason"));
+                $host, $this->prefs['port'], $errstr ? $errstr : "Unknown reason"));
 
             return false;
         }