Fix sandbox_linux_unittests with UBSan

Casting pointers like this is UB if the pointer isn't aligned. It's also
a strict aliasing violation. Instead, load with memcpy.

Bug: 1394755
Change-Id: Icdbd9bda02aec3959703f67160af50c151f398a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4974540
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214518}