Add another check in profiler's stack iterator to ensure stability.

Add another check in profiler's stack iterator to ensure stability.
I made a test run of benchmarks and found another weak point. After
fixing it, benchmarks are able to run for a 100 times in a row
successfully in both builds.

Committed: http://code.google.com/p/v8/source/detail?r=1612

[Kasper Lund, 2009-03-25 14:03:56]

LGTM as long as is_at_function() is only called from the safe iterator which I
think is the case.

[Mikhail Naganov, 2009-03-25 14:07:12]

On 2009/03/25 14:03:56, Kasper Lund wrote:
> LGTM as long as is_at_function() is only called from the safe iterator which I
> think is the case.

Yes, you are right. It was added specifically for it.