Passing a zero length key (or secret) gives no safety against XSRF
attacks. This is a relatively easy mistake to make, e.g. by passing
`make([]byte, 0, 1024)` to `rand.Read` instead of `make([]byte, 1024)`,
and currently fails open, silently.

This uses panic, as the API does not allow returning a structured error,
and catching this programming error is not worth breaking API
compatibility. Passing a zero length secret is also not an error
condition that API callers would handle, so there is little value in
returning a proper error.

Change-Id: Ib6457347675872188d51d2a220eee4b67900f79e
Reviewed-on: https://go-review.googlesource.com/42411Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>