fix bug when searching in forums : if more than one forum or one categor...

Hi, 

a client of mine has just indicated me a bug...
Conditions : 
- logged as student
- 2 forums in a course

When we try to search a topic of the second forum, it gives no result, due to the $row['forum_id']['forum_category'] statement which returns in fact the forum id and not the category id.
Here is a pull request to fix it.