fix(select): Fix NPE by checking for controller existence.

In some cases, the select could be removed from the DOM before some
of the cleanup code was executed. This caused a `.controller()` call
to return `null`, but we did not check for existence before using
the controller which resulted in null pointer exception.

Fixes #7079.

Closes #7560

Author: topherfangio

src/components/select/select.js

@@ -1280,7 +1280,7 @@ function SelectProvider($$interimElementProvider) {
       var mdSelect = opts.selectCtrl;
       if (mdSelect) {
         var menuController = opts.selectEl.controller('mdSelectMenu');
-        mdSelect.setLabelText(menuController.selectedLabels());
+        mdSelect.setLabelText(menuController ? menuController.selectedLabels() : '');
         mdSelect.triggerClose();
       }
     }