Rubicon OpenRTB crashes if banner.format is undefined

[dbemiller]

To reproduce, boot PBS and POST the following request to /openrtb2/auction:

{
  "id": "e8f0d8a3-f360-43c2-af08-34aa265108b0",
  "site": {
    "page": "mysite.com"
  },
  "imp": [
    {
      "id":"/19968336/header-bid-tag-0",
      "banner": {
        "w": 350,
        "h":200
      },
      "ext": {
        "rubicon": {
          "accountId": 1001,
          "siteId":113932,
          "zoneId":535510
        }
      }
    }
  ]
}

The app crashes with the following trace:

panic: runtime error: makeslice: cap out of range

goroutine 16 [running]:
github.com/prebid/prebid-server/adapters/rubicon.parseRubiconSizes(0x0, 0x0, 0x0, 0x17, 0x40, 0x0, 0x0, 0x4d00000, 0x0)
	/Users/dbemiller/go/src/github.com/prebid/prebid-server/adapters/rubicon/rubicon.go:211 +0x5c
github.com/prebid/prebid-server/adapters/rubicon.(*RubiconAdapter).MakeRequests(0xc459f61380, 0xc459f89a40, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/dbemiller/go/src/github.com/prebid/prebid-server/adapters/rubicon/rubicon.go:576 +0x118e
github.com/prebid/prebid-server/exchange.(*bidderAdapter).requestBid(0xc459fbdee0, 0x491b660, 0xc4201bf7a0, 0xc459f89a40, 0x0, 0x4622b03, 0x7, 0x0, 0x0, 0x0, ...)
	/Users/dbemiller/go/src/github.com/prebid/prebid-server/exchange/bidder.go:82 +0x58
github.com/prebid/prebid-server/exchange.(*exchange).getAllBids.func1(0xc459f61340, 0x491b660, 0xc4201bf7a0, 0xc45a07ba70, 0x0, 0xc4201bf8c0, 0x4622b03, 0x7)
	/Users/dbemiller/go/src/github.com/prebid/prebid-server/exchange/exchange.go:134 +0x194
created by github.com/prebid/prebid-server/exchange.(*exchange).getAllBids
	/Users/dbemiller/go/src/github.com/prebid/prebid-server/exchange/exchange.go:129 +0x153