Check that the tag GET variable is a string

[hannob]

By sending a tag variable that is an array one can trigger an uncaught exception. I saw these in my logs, likely due to some internet scans.

This can be reproduced like this:

curl 'https://[host]/rss.php?serendipity\[tag\]\[\]=1'

Fixing this by checking that the var is actually a string, and ignore it otherwise.

[onli]

Merged, thanks! I added the changelog in bf35651 (would be ideal if it could be added directly next time :) )