dns/dnsmessage: add boundary check in unpackSVCBResource

Currently, bodyEnd is calculated using the length parameter from the
resource header without verifying if it exceeds the physical capacity
of the msg buffer.

If a malformed record provides a length that
exceeds the buffer, it bypasses the first-pass parameter validation
and causes an out-of-bounds slice during the second-pass copy.

Adding a check against len(msg) aligns this function with the boundary
enforcement used throughout the rest of the package.

Change-Id: I13f6ca83d1c30eac02286a49c12f8ec543d33e41
GitHub-Last-Rev: 78c35a160c45f09f2db04d9ec076dfb091a15595
GitHub-Pull-Request: golang/net#249
Reviewed-on: https://go-review.googlesource.com/c/net/+/781880
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: ISMAIL GAMAL <ismailismailgamal52@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>