If DevTools is turned on in the middle of doing some things with async
functions, then more items may be popped from the Promise stack than were
pushed to it. In this sort of case, it's OK to have a catch misprediction,
but a crash is unacceptable. This patch defensively handles this edge
case where the Promise stack is unexpectedly empty for that reason.

BUG=v8:5167

Review-Url: https://codereview.chromium.org/2361333003
Cr-Commit-Position: refs/heads/master@{#39705}