omapfb: Fix potential NULL pointer dereference in kmalloc

Memory allocated, using kmalloc, for new_compat may fail. This patch
checks for such an error and prevents potential NULL pointer
dereference.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Cc: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>