Fix crasher when checking for "of", but next token has no literal buffer

Also fix a typo in an assertion in scanner.h.

R=mstarzinger@chromium.org
BUG=248025
TEST=mjsunit/regress/regress-crbug-248025.js

Review URL: https://codereview.chromium.org/16549003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15059 ce2b1a6d-e550-0410-aec6-3dcde31c8c00