This CL fixes a parser crash in REPL mode. Some SyntaxErrors can cause
the AST to contain NULL nodes, resulting in a crash when we want to
rewrite the AST after parsing.

Instead of re-writing a broken AST we bail early.

R=leszeks@chromium.org

Bug: chromium:1040034, chromium:1045758
Change-Id: I9c559f6de5969c8db17833ccbdb1608627b46311
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2023547Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66008}