An error in string slice offsets caused the loop to run forever if the
first character in the argument was a period.

Fixes #10833.

Change-Id: Iefb6aac5cff8864fe93d08e2600cb07d82c6f6df
Reviewed-on: https://go-review.googlesource.com/10285Reviewed-by: Russ Cox <rsc@golang.org>