Don't use an unsanitized user-defined URL as a regular expression.

User-defined URLs should not be used as regular expressions without being sanitized - which is ironically the kind of thing Nikto is looking for in the first place.  :-)  Without this change, a URL like '/[blah.txt' would cause a crash due to the URL itself being an invalid regular expression pattern.  Tyler Farr assisted with finding this problem and devising a fix.

It was pretty obvious in the code that it meant to remove instances of the URL from the content anyway, but since it wasn't being escaped, URLs were being used as expressions instead.