ion: ion_system_heap: Fix null pointer dereference

The pointer returned from alloc_pages passed unchecked to
ion_pages_sync_for_device.
This patch checks the return for NULL.

Change-Id: I407efc73a1e1297f382a7926cc79542bde88d06d
Fixes: 2d4fdc7bde46 ("ion: Fix DMA operations for ARM64")
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>