Assert backing_file should exist before attempting to create it

Fixes bug 1200249

Change-Id: If7878361711f835247aaf64ce26f0058f8c4b5ba
(cherry picked from commit 879565efe51f19bd6f4d69d1b430b995f62106c8)
Merge "Assert backing_file should exist before attempting to create it" into stable/grizzly