Fix using unix:///path/to/socket.file in memcached driver (#7210)

off by one when stripping the memcache (sans d) compatible AF_UNIX prefix

program/lib/Roundcube/cache/memcached.php

@@ -107,7 +107,7 @@ class rcube_cache_memcached extends rcube_cache
                     if (!$port) $port = 11211;
                 }
                 else {
-                    $host = substr($host, 8);
+                    $host = substr($host, 7);
                     $port = 0;
                 }