During bootstrapping, both the kFreeSpaceMap root and the map of
actual FreeSpace objects can be nullptr, so the respective DCHECK
must be robust towards this.

Change-Id: Ic7b66b5ab40d7b1bf77c2de936e611e0836cd51f
Reviewed-on: https://chromium-review.googlesource.com/c/1382094Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58314}