Parser::parseVariableDeclarationList should null check the node before attempting...

Parser::parseVariableDeclarationList should null check the node before attempting to create a new CommaExpr
https://bugs.webkit.org/show_bug.cgi?id=154244
rdar://problem/24290670

Reviewed by Michael Saboff.

* parser/ASTBuilder.h:
(JSC::ASTBuilder::appendToCommaExpr): Catch the bug sooner in debug.
* parser/Parser.cpp:
(JSC::Parser<LexerType>::parseVariableDeclarationList): Fix the bug.
* tests/stress/for-let-comma.js: Added. This used to crash in debug and release.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@196587 268f45cc-cd09-0410-ab3c-d52691b4dbfc