Fix PHP warning when handling shared namespace with empty prefix (#5420)

CHANGELOG

@@ -49,6 +49,7 @@ CHANGELOG Roundcube Webmail
 - Fix so "All" messages selection is resetted on search reset (#5413)
 - Fix bug where folder creation could fail if personal namespace contained more than one entry (#5403)
 - Fix error causing empty INBOX listing in Firefox when using an URL with user:password specified (#5400)
+- Fix PHP warning when handling shared namespace with empty prefix (#5420)
 
 RELEASE 1.2.1
 -------------

program/steps/settings/folders.inc

@@ -330,7 +330,7 @@ function rcmail_subscription_form($attrib)
             if (!$disabled) {
                 $tmp_ns = array_merge((array)$namespace['other'], (array)$namespace['shared']);
                 foreach ($tmp_ns as $item) {
-                    if (strpos($folder['id'], $item[0]) === 0) {
+                    if (strlen($item[0]) && strpos($folder['id'], $item[0]) === 0) {
                         $disabled = true;
                         break;
                     }