Fix crash with v8-bench.js

Fix crash with v8-bench.js

Sometimes the arguments array of the context is provided by the caller and not
copied. In that case the address may not be valid anymore after the call, so
we need to reset arguments/argumentCount after the call in popContext, otherwise
we end up calling mark() on dangling pointers.

Also make sure to mark the entire arguments array, not too much and not too little,
i.e. use argumentCount.

Change-Id: I8b96521b0fb15142ed8c723a9354d275be48cc58
Reviewed-by: Lars Knoll <lars.knoll@digia.com>