Fix for empty bbcode uid

Hi,

Apply check for empty bbcode uid to prevent all : (colons) being removed.

Example:

    // remove the bbcode_uid from post_text
    $topic->post_text = preg_replace("/:$topic->bbcode_uid/", '', $topic->post_text);

If $topic->bbcode_uid is empty, then all colons are removed, fx in http:// -> http// .

So possible fix is this

    // remove the bbcode_uid from post_text
    if(!empty($topic->bbcode_uid)) {
       $topic->post_text = preg_replace("/:$topic->bbcode_uid/", '', $topic->post_text);
    }

I found this issue on the following lines; 358, 371, 602, 780

Those line numbers are approx, due to some other bugfixes i have applied in #594876: Various bugfixes for signatures, access time and attachments