Avoid member access within null pointer in PersistentMemoryAllocator

Create temporary objects for the assertions instead of doing UB.

Bug: 966850
Change-Id: I344622fcf0e616be313a510345e0d6cf65d22113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628723
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Maciej Pawlowski <mpawlowski@opera.com>
Cr-Commit-Position: refs/heads/master@{#663524}
1 file changed