Safe stack iterator is supposed to work even when the stack is in an inconsistent state.
E.g. during cpu profile sample recording. This patch eliminates a crash if the frame marker
is found to be bogus.

BUG=v8:4705
LOG=N

Review URL: https://codereview.chromium.org/1633323002

Cr-Commit-Position: refs/heads/master@{#33558}