It's quite possible for DebugInfos to exist without the presence of a
bytecode array, since DebugInfos are created for all functions for which
we have a CoverageInfo. Free such objects properly.

Also move the corresponding deletion of CoverageInfos on unload up
before the early exit.

Bug: v8:6000
Change-Id: Idde45b222290aa8b6828b61ff2251918b8ed2aed
Reviewed-on: https://chromium-review.googlesource.com/664811Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48024}