HTTP Crawler: don't expect page object for msg

The `crawler_process_page` method in HttpCrawler assumes that the
`page` object passed into the method is not nil when formatting the
`msg` string for printing to console.
Address the assumption with a ternary check leaving the `|| "ERR"`
handling for `page.code` itself being nil inside the assignment
when page is not nil.

Testing:
 `Error accessing page undefined method '[]' for nil:NilClass` is
no longer being thrown when scanning an odd HTTP service.

Author: RageLtMan

lib/msf/core/auxiliary/http_crawler.rb

@@ -243,7 +243,7 @@ def crawl_target(t)
   # Specific module implementations should redefine this method
   # with whatever is meaningful to them.
   def crawler_process_page(t, page, cnt)
-    msg = "[#{"%.5d" % cnt}/#{"%.5d" % max_page_count}]    #{page.code || "ERR"} - #{@current_site.vhost} - #{page.url}"
+    msg = "[#{"%.5d" % cnt}/#{"%.5d" % max_page_count}]    #{page ? page.code || "ERR" : "ERR"} - #{@current_site.vhost} - #{page.url}"
     case page.code
       when 301,302
         if page.headers and page.headers["location"]