Avoid coping the last character to a local variable, by checking the parsing
state in a different order.

BUG=v8:7926

Change-Id: I0b62f711674beac8c81a25dd566a5ed0d681948b
Reviewed-on: https://chromium-review.googlesource.com/1148456
Commit-Queue: Florian Sattler <sattlerf@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54657}