internal/poll: better panic for invalid write return value

internal/poll: better panic for invalid write return value

For #61060

Change-Id: I13cd73b4062cb7bd248d2a4afae06dfa29ac0203
Reviewed-on: https://go-review.googlesource.com/c/go/+/577955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>