Return output

When $use_gd is false, returning $output so that the captcha images that have been generated are actually embedded into the final rendering of the page.

Fixes #536.

plugins/serendipity_event_spamblock/serendipity_event_spamblock.php

@@ -842,6 +842,7 @@ class serendipity_event_spamblock extends serendipity_event
                 );
             }
             $output .= '</div>';
+            return $output;
         }
     }
 
@@ -1696,4 +1697,4 @@ class serendipity_event_spamblock extends serendipity_event
 }
 
 /* vim: set sts=4 ts=4 expandtab : */
-?>
+?>