strings: avoid unnecessary variable setting

strings: avoid unnecessary variable setting

We initialize fieldStart to 0, then set it to i without ever reading
0, so we might as well just initialize it to i.

Change-Id: I17905b25d54a62b6bc76f915353756ed5eb6972b
Reviewed-on: https://go-review.googlesource.com/52933
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>