This was found while fuzzing another program, triggering a panic in
x509.ParseECPrivateKey.

Fixes #11154

Change-Id: Ief35ead38adf14caec4d37b9eacf8a92e67cd1e6
Reviewed-on: https://go-review.googlesource.com/10712Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>