The catch prediction logic got confused when we merged the reactions and
result fields of JSPromise, because for settled promises it would start
to treat the result as reactions list, leading to a crash most likely or
memory corruption in the worst case (only if break on uncaught exception
is enabled). We can only inspect reactions when the promise is still in
"pending" state.

Bug: chromium:808973, v8:7253
Change-Id: I15162c96fce959a052fbc628addd9418da39327c
Reviewed-on: https://chromium-review.googlesource.com/903163Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51106}