* [jsfm] fix undefined for element's depth

[terrykingcha]

for a example

<template>
  <container append="tree">
    <text repeat="{{list}}">{{text}}</text>
  </container>
</template>

<script>
module.exports = {
  data: {
    list: []
  },
  created: function() {
    someAsyncMethod(function() {
       this.list.push({text: 123})
    })
  }
}
</script>

will cause undefined for fragBlock.element.depth when calling _watchBlock in lib/vm/compiler.js.

so, just set 0 by default to fix it.

[Jinjiang]

Btw, the branch format should be jsfm-bugfix-element-depth. jsfm first and no upper case, no underline.