don't invoke sed when no tag given

the bug was that sed was expecting file content on stdin when didn't receive
any filename, and script was "frozen" when someone tried to edit a post
without any tags.

bb.sh

@@ -784,6 +784,7 @@ tags_in_post() {
 # Arguments are tags
 # Prints one line with space-separated tags to stdout
 posts_with_tags() {
+    [ $# -lt 1 ] && return
     tag_files="$(echo "$@" | sed "s/\S\+/tag_&.html/g")"
     sed -n '/^<h3><a class="ablack" href="[^"]*">/{s/.*href="\([^"]*\)">.*/\1/;p}' $tag_files
 }