silence has_changed_character may be used uninitialized warning

Some slightly older versions of gcc aren't clever enough to figure out
that this variable won't be used if the return value from the function
that initializes it is false.  Tested with gcc 4.5.1.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/99553005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18456 ce2b1a6d-e550-0410-aec6-3dcde31c8c00