strip trailing line in higlighted code

The current code add 2 unwanted trailing lines in highlighted code. One is produced
by DokuWiki's syntax and is fixed by this patch. The second one is created somewhere
inside GeShi. A bug report was submitted upatream [1]

[1] http://sourceforge.net/tracker/index.php?funcÞtail&aid31373&group_id4997&atidg0231

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