Fix threads tab complete bug

When tab completing the threads command with 2 arguments, msfconsole will crash if the first is not in the list of valid arguments.

Before this patch:
```
msf-git (S:0 J:0)> threads foobar /home/steiner/repos/msf-git/lib/msf/ui/console/command_dispatcher/core.rb:1028:in `cmd_threads_tabs': undefined method `[]' for nil:NilClass (NoMethodError)
	from /home/steiner/repos/msf-git/lib/rex/ui/text/dispatcher_shell.rb:357:in `tab_complete_helper'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/dispatcher_shell.rb:317:in `block in tab_complete_stub'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/dispatcher_shell.rb:306:in `each'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/dispatcher_shell.rb:306:in `tab_complete_stub'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/dispatcher_shell.rb:291:in `tab_complete'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/shell.rb:59:in `block in init_tab_complete'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/readline.rb:136:in `call'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/readline.rb:136:in `readline_attempted_completion_function'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:6277:in `gen_completion_matches'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:6761:in `rl_complete_internal'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:6851:in `rl_complete'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:4322:in `_rl_dispatch_subseq'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:4311:in `_rl_dispatch'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:4727:in `readline_internal_charloop'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:4801:in `readline_internal'
	from /home/steiner/.rvm/gems/ruby-2.1.4@metasploit-framework/gems/rb-readline-0.5.1/lib/rbreadline.rb:4823:in `readline'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/input/readline.rb:132:in `readline_with_output'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/input/readline.rb:86:in `pgets'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/shell.rb:184:in `run'
	from /home/steiner/repos/msf-git/lib/metasploit/framework/command/console.rb:30:in `start'
	from /home/steiner/repos/msf-git/lib/metasploit/framework/command/base.rb:82:in `start'
	from ./msfconsole:48:in `<main>
```

Validation:
- [x] Start msfconsole
- [x] Type in ```threads show options``` and hit tab twice
- [x] Don't get kicked out of msfconsole with a stack trace