net/http: respond with 400 Bad Request for empty hex number of chunk length

net/http: respond with 400 Bad Request for empty hex number of chunk length

Fixes #64517

Change-Id: I78b8a6a83301deee05c3ff052a6adcd1f965aef2
Reviewed-on: https://go-review.googlesource.com/c/go/+/553835
Auto-Submit: Damien Neil <dneil@google.com>
Commit-Queue: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>