Fix the other places the slice length was being believed, and refactor
the code to use a single function to unify the check.

Fixes #10273.

Change-Id: Ia62b25203fbe87c95d71a70ebc1db8d202eaa4a4
Reviewed-on: https://go-review.googlesource.com/8511Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>