string is larger than 2047 chars we will encode the slice as two Smis
instead of one.  The calculation of the max size of the string builder
did not take this into account.

BUG=http://code.google.com/p/v8/issues/detail?id=515
Review URL: http://codereview.chromium.org/402056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00