at91sam9: fix broken error propagation

Change-Id: I3288ce6f7642e519c26dd099be888a395fe1de94
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/159
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/flash/nand/at91sam9.c

@@ -348,6 +348,7 @@ static uint8_t * at91sam9_oob_init(struct nand_device *nand, uint8_t *oob, uint3
 		oob = malloc(*size);
 		if (!oob) {
 			LOG_ERROR("Unable to allocate space for OOB");
+			return NULL;
 		}
 
 		memset(oob, 0xFF, *size);