getActiveEditor returns undefined if no window is open

* lib/symbols-view.coffee:45 attempts to grab the current editor
  and then feed the cursorBufferPosition into var previous.
  If there is no current editor open, an undefined error is thrown
  on getCursorBufferPosition when trying to assign var previous.

* Add an if statement to check the editor before calling
  getCursorBufferPosition() on an undefined object.

 * Resolves #29.