A module context holds the module object in its extension slot, which should not be recursed on (it's not even a JSObject).

(I don't know how this code path can be triggered, though.)

R=adamk@chromium.org
BUG=v8:1569

Review-Url: https://codereview.chromium.org/2919063002
Cr-Commit-Position: refs/heads/master@{#45695}