x86: OLPC: have prom_early_alloc BUG rather than return NULL

..similar to what sparc's prom_early_alloc does.
Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/x86/platform/olpc/olpc_dt.c

@@ -140,8 +140,7 @@ void * __init prom_early_alloc(unsigned long size)
 		 * wasted bootmem) and hand off chunks of it to callers.
 		 */
 		res = alloc_bootmem(chunk_size);
-		if (!res)
-			return NULL;
+		BUG_ON(!res);
 		prom_early_allocated += chunk_size;
 		memset(res, 0, chunk_size);
 		free_mem = chunk_size;