The semaphore.Weighted API accepts int64 for weights. If a negative
weight is passed, it mathematically corrupts the internal state tracker
(s.cur) and bypasses the package's existing safety checks.

This adds strict boundary validation to panic immediately if a negative
weight is provided.

Fixes golang/go#80183

Change-Id: I7e74bad404a8b99aef7e3d2189dda43f8ea644c2
GitHub-Last-Rev: ee6289c356f4626ebca52dbe86d856cc713d7be0
GitHub-Pull-Request: golang/sync#31
Reviewed-on: https://go-review.googlesource.com/c/sync/+/796080
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Zain Yousef <zain.19dj@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>