The implication was actually in the wrong direction: If there is no
memory start address, then the size must be 0.
If the size is 0 though, we might allocate nevertheless to have guard
pages around the accessible memory.

R=ahaas@chromium.org
BUG=chromium:736584

Change-Id: I297dece658d5eaf69c58ecb109ff21d3ca0b8a8d
Reviewed-on: https://chromium-review.googlesource.com/548635Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46221}