search API: elapsedMilliseconds overflows when no search results are returned

Created by: nicksnyder

From enterprise created by slimsag : sourcegraph/enterprise#12625

If you perform a request that returns no search results (i.e. one that returns almost instantly), the elapsedMilliseconds field returned to your will be 2077252342 (577h0m52.342s):

https://sourcegraph.com/api/console#%7B%22query%22%3A%22query+(%24query%3A+String!)+%7B%5Cn++search(query%3A+%24query)+%7B%5Cn++++results+%7B%5Cn++++++resultCount%5Cn++++++approximateResultCount%5Cn++++++elapsedMilliseconds%5Cn++++%7D%5Cn++%7D%5Cn%7D%5Cn%22%2C%22variables%22%3A%22%7B%5Cn++%5C%22query%5C%22%3A+%5C%22abc%5C%22%5Cn%7D%22%7D

This caused me to waste a lot of time because I thought I was using the returned value incorrectly somehow, but it turned out that wasn't the case. Users of our API will run into this as well.