[AGPGART] drivers/char/agp/sgi-agp.c: check kmalloc() return value

drivers/char/agp/sgi-agp.c: check kmalloc() return value

Signed-off-by: Amit Choudhary <amit2030@gmail.com>
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>