crypto/internal/fips/hkdf: correctly set the service indicator for short salts

crypto/internal/fips/hkdf: correctly set the service indicator for short salts

For #69536

Change-Id: Ibe2623311c8be5fb3e7411b33e61bf66d026e14d
Reviewed-on: https://go-review.googlesource.com/c/go/+/626877
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>