Fix #4646 - get_module_resource should check nil before using get_resource

Fix #4646. The get_module_resource needs to check nil first before
using the get_resource method (from HttpServer)

lib/msf/core/exploit/remote/browser_exploit_server.rb

@@ -106,7 +106,7 @@ module Msf
     # @return [String] URI to the exploit page
     #
     def get_module_resource
-      "#{get_resource.chomp("/")}/#{@exploit_receiver_page}/"
+      "#{get_resource.to_s.chomp("/")}/#{@exploit_receiver_page}/"
     end
 
     #