Fix jobs NilClass tab complete bug

The ```jobs``` command seems to suffer from the same NilClass bug that the ```threads``` command did that was addressed in #4418. When jobs is entered and tab completed with invalid values it exists msfconsole with a stack trace.

Before patch:
```
msf-git (S:0 J:0)> jobs show op/home/steiner/repos/msf-git/lib/msf/ui/console/command_dispatcher/core.rb:876:in `cmd_jobs_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/repos/msf-git/lib/rex/ui/text/input/readline.rb:146:in `call'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/input/readline.rb:146:in `readline'
	from /home/steiner/repos/msf-git/lib/rex/ui/text/input/readline.rb:146: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>'
```

Verification steps:
- [ ] Start msfconsole
- [ ] Type in ```jobs show op``` and hit tab twice
- [ ] Don't get kicked out of msfconsole with a stack trace

Did a quick grep for ```_opts.fmt\[\S+\[1\]\]``` to look for other occurrences and the only two instances I found are in jobs and threads which will be fixed once this is landed.