Don't clear the selection if it's empty

This avoids emitting selection change events when they aren't really needed. This extra event was causing the highlighted line to be updated when it didn't need to be.

src/app/selection.coffee

@@ -87,7 +87,8 @@ class Selection
     @editSession.buffer.getTextInRange(@getBufferRange())
 
   clear: ->
-    @anchor?.destroy()
+    return unless @anchor
+    @anchor.destroy()
     @anchor = null
     @screenRangeChanged()