inch should be lenient with malformed YARD @overload parameters

I ran `inch` against a problem with malformed YARD @overload parameters and got this stack trace:

```
/Volumes/TempStorage/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/inch-0.4.0/lib/inch/code_object/provider/yard/object/method_object.rb:126:in `block in overloaded_parameter_names': undefined method `map' for nil:NilClass (NoMethodError)
	from /Volumes/TempStorage/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/inch-0.4.0/lib/inch/code_object/provider/yard/object/method_object.rb:125:in `map'
	from /Volumes/TempStorage/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/inch-0.4.0/lib/inch/code_object/provider/yard/object/method_object.rb:125:in `overloaded_parameter_names'
	from /Volumes/TempStorage/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/inch-0.4.0/lib/inch/code_object/provider/yard/object/method_object.rb:145:in `signature_parameter_names'
	from /Volumes/TempStorage/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/inch-0.4.0/lib/inch/code_object/provider/yard/object/method_object.rb:90:in `all_parameter_names'
```

From a method incorrectly documented like:

```ruby
  # @overload 
  #   @param [Symbol] an i18n key 

def do_something *args
end
```

It'd be nice if inch reported this better, but silently ignoring the problem (and just taking the doc coverage hit) seemed straightforward enough.