fix($animate): ensure that animateable directives cancel expired leave animations

If enter -> leave -> enter -> leave occurs then the first leave animation will
animate alongside the second. This causes the very first DOM node (the view in ngView
for example) to animate at the same time as the most recent DOM node which ends
up being an undesired effect. This fix takes care of this issue.

Closes #5886
fix(ngAnimate): TypeError Cannot call method 'querySelectorAll' in cancelChildAnimations

When an element containing both ng-repeat and ng-if directives attempts to remove any items from
the repeat collection, the following error is thrown: "TypeError Cannot call method 'querySelectorAll'
of undefined".  This happens because the cancelChildAnimations code naively belives that the jqLite
object always has an element node within it. The fix in this commit addresses to securely check to see
if a node was properly extracted before any child elements are inspected.

Closes #6205