NewWeighted currently accepts a negative capacity, creating a
semaphore for which even a zero-weight acquisition cannot succeed.
Reject negative capacities at construction time while continuing to
allow zero capacity.

Updates golang/go#80183

Change-Id: Id819efed370bb81e8bf13891d781f90e298a2a07
Reviewed-on: https://go-review.googlesource.com/c/sync/+/808920
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: race quite <quiterace@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: David Chase <drchase@google.com>