Only attempt to hide the dropdown if it's showing

[rbright]

There are multiple events that trigger the results_hide function.

1. Expand a dropdown and close it again. This triggers results_hide through the results_toggle function. Note that the dropdown is still highlighted.
2. Focus on another element. This triggers results_hide through the close_field function.

Suppose we have two Chosen dropdowns.

Expanding the first dropdown sets @results_showing to true. Closing it sets the flag to false. However, expanding the second dropdown will first toggle the flag to true but then toggle it back to false when results_hide is called as a result of close_field.

An example of this behavior can be seen at http://www.getharvest.com/platform.