Land rapid7#4418, threads tab completion NilClass fix

Author: wvu

lib/msf/ui/console/command_dispatcher/core.rb

@@ -1025,7 +1025,7 @@ def cmd_threads_tabs(str, words)
       return @@threads_opts.fmt.keys
     end
 
-    if @@threads_opts.fmt[words[1]][0] and (words.length == 2)
+    if words.length == 2 and (@@threads_opts.fmt[words[1]] || [false])[0]
       return framework.threads.each_index.map{ |idx| idx.to_s }
     end