The main fix is to mark stack memory the SafeStackFrameIterator
accesses as initialied.

Drive-by: Make sure we bail out when the simulator is in the
process of updating FP/SP registers.

BUG=v8:5156

Review-Url: https://codereview.chromium.org/2104763002
Cr-Commit-Position: refs/heads/master@{#37315}