Also add a DCHECK to a branch that can only be taken for the null
prototype.

R=sigurds@chromium.org

Change-Id: Ib94fe8f25ecfd1a4baa576915e6edfa60bcd771b
Reviewed-on: https://chromium-review.googlesource.com/1109961
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53939}