Add a missing check for a prototype to GetDerivedMap.

Bug: chromium:999310
Change-Id: I99c342a53e3b95bb7b624ff14c1c40576ee629df
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776092
Auto-Submit: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63473}