The bug occurred when we detected an erroneous char late, and put the last
character in a chunk into the "incomplete char" buffer. It was not correctly
retrieved when seeking.

BUG=v8:6836

Change-Id: I8ca946dfdb39244c5ca0bdcebe047047010b3a07
Reviewed-on: https://chromium-review.googlesource.com/670729
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48066}