Avoid pointer underflow in CopyCharsUnsigned.

R=dcarney@chromium.org
BUG=v8:2493

Review URL: https://chromiumcodereview.appspot.com/11961012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13398 ce2b1a6d-e550-0410-aec6-3dcde31c8c00