Script streaming: fix split UTF-8 character handling.

Invalid UTF-8 data can contain too many characters which look like they're part
of a multi-byte character, and that was overflowing a buffer.

BUG=chromium:417891
LOG=n

.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/607043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24251 ce2b1a6d-e550-0410-aec6-3dcde31c8c00