Assert that this err is nil
Assert that this err is nil

The return value of Update in this test case is nil, so we should assert
that it's actually nil, and not some other unexpected error. This also
avoids an apparent code generation bug on fuchsia, that trips over a
segfault in the reflection library when it's trying to reflect the
type of a `var err error` when `err` is `nil`.

Change-Id: I03eae3ba99de3cc4b27ad929991b755ffc484be7

theupdateframework/go-tuf