fix(tooltip): guard against missing offsetParent.

Author: jelbourn

src/components/tooltip/tooltip.js

@@ -239,12 +239,13 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $mdUtil, $mdThe
       var tipRect = $mdUtil.offsetRect(element, tooltipParent);
       var parentRect = $mdUtil.offsetRect(parent, tooltipParent);
       var newPosition = getPosition(scope.direction);
+      var offsetParent = element.prop('offsetParent');
 
       // If the user provided a direction, just nudge the tooltip onto the screen
       // Otherwise, recalculate based on 'top' since default is 'bottom'
       if (scope.direction) {
         newPosition = fitInParent(newPosition);
-      } else if (newPosition.top > element.prop('offsetParent').scrollHeight - tipRect.height - TOOLTIP_WINDOW_EDGE_SPACE) {
+      } else if (offsetParent && newPosition.top > offsetParent.scrollHeight - tipRect.height - TOOLTIP_WINDOW_EDGE_SPACE) {
         newPosition = fitInParent(getPosition('top'));
       }