[stitcher] Context is not finished when mixed vast / assets payload

[matvp91]

The following stitcher payload gives an error:

{
  "uri": "c7e200e9-6f99-46a5-abad-f8530e0c44c5",
  "interstitials": [
    {
      "time": 10,
      "assets": [
        {
          "uri": "a4d0ad2b-6d37-4ad5-8430-6a26504fb3b"
        }
      ]
    },
    {
      "time": 20,
      "vast": {
        "url": "https://pubads.g.doubleclick.net/gampad/ads?iu=/21775744923/external/single_ad_samples&sz=640x480&cust_params=sample_ct%3Dlinear&ciu_szs=300x250%2C728x90&gdfp_req=1&output=vast&unviewed_position_start=1&env=vp&impl=s&correlator={random}"
      }
    }
  ]
}