fix(dateLocale): guard for setting midnight to null. Fixes #4780

Author: jelbourn

src/components/datepicker/dateUtil.js

@@ -195,7 +195,9 @@
      * @param {Date} date
      */
     function setDateTimeToMidnight(date) {
-      date.setHours(0, 0, 0, 0);
+      if (isValidDate(date)) {
+        date.setHours(0, 0, 0, 0);
+      }
     }
 
     /**

src/components/datepicker/dateUtil.spec.js

@@ -304,6 +304,12 @@ describe('$$mdDateUtil', function() {
     expect(dayAtMidnight.getMilliseconds()).toBe(0);
   });
 
+  it('should not error when trying to set an invalid date to midnight', function() {
+    dateUtil.setDateTimeToMidnight(new Date(NaN));
+    dateUtil.setDateTimeToMidnight(null);
+    dateUtil.setDateTimeToMidnight(undefined);
+  });
+
   it('should determine whether dates are valid', function() {
     expect(dateUtil.isValidDate(null)).toBeFalsy();
     expect(dateUtil.isValidDate(undefined)).toBeFalsy();