fix(tooltip): using tooltip inside subheader causes Firefox hang.

When a `<md-tooltip>` was placed inside the `<md-subheader>`,
an infinite loop was being hit (only in Firefox) which was
causing the browser to hang.

The infinite loop was caused by the tooltip's
`getParentWithPointerEvents()` method which travels up the DOM looking for a parent who has pointer events. In Firefox, the `element.parent()` can apparently return an empty jqLite object that is not null, but whose length is 0. In this case, calling `parent.parent()` again will return the same object.

Add a check to the tooltip's loop to ensure that `parent.length` is never 0.

Fixes #4777. Closes #4800.

src/components/tooltip/tooltip.js

@@ -106,9 +106,12 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $mdUtil, $mdThe
      */
     function getParentWithPointerEvents () {
       var parent = element.parent();
-      while (parent && hasComputedStyleValue('pointer-events','none', parent[0])) {
+
+      // jqLite might return a non-null, but still empty, parent; so check for parent and length
+      while (hasComputedStyleValue('pointer-events','none', parent)) {
         parent = parent.parent();
       }
+
       return parent;
     }
 
@@ -123,12 +126,17 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $mdUtil, $mdThe
 
 
     function hasComputedStyleValue(key, value, target) {
-      key    = attr.$normalize(key);
-      target = target || element[0];
+      var hasValue = false;
 
-      var computedStyles = $window.getComputedStyle(target);
+      if ( target && target.length  ) {
+        key    = attr.$normalize(key);
+        target = target[0] || element[0];
+
+        var computedStyles = $window.getComputedStyle(target);
+        hasValue = angular.isDefined(computedStyles[key]) && (computedStyles[key] == value);
+      }
 
-      return angular.isDefined(computedStyles[key]) && (computedStyles[key] == value);
+      return hasValue;
     }
 
     function bindEvents () {