The {last_end_found_} field is redundant. If it's false, there will be
a control struct left on the stack.
Thus remove that field, and simplify the error detection condition.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: I16ac4597f229c5e3abd923f8eb504f93afb82eb4
Reviewed-on: https://chromium-review.googlesource.com/c/1373788Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58198}