From: Subash Patel <subash.rp@samsung.com>

exynos_pages_to_sg() internally calls sg_kmalloc() which can return
no pages when the system is under high memory crunch. One such instance
is chromeos-install in the chromeos. This patch adds check for the return
value of the function in subject to return NULL on failure.

BUG=chrome-os-partner:9481
TEST=built, ran on snow and tried chromeos-install without a crash

Change-Id: I0abda74beaedae002a17de9962d7a462a2a7c2fb
Signed-off-by: Subash Patel <subash.rp@samsung.com>