Harden JSStringJoiner
https://bugs.webkit.org/show_bug.cgi?id=112093

Reviewed by Filip Pizlo.

Harden JSStringJoiner, make it use our CheckedArithmetic
class to simplify everything.

* runtime/JSStringJoiner.cpp:
(JSC::JSStringJoiner::build):
* runtime/JSStringJoiner.h:
(JSStringJoiner):
(JSC::JSStringJoiner::JSStringJoiner):
(JSC::JSStringJoiner::append):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145594 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Appease MSVC

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145595 268f45cc-cd09-0410-ab3c-d52691b4dbfc