s390/setup: call memblock_reserve only for size > 0

reserve_initrd currently calls memblock_reserve even if the to be
reserved size is zero. Even though the memblock core code can handle
this correctly, it still yields confusing debug messages if
memblock debugging is enabled.
Therefore make sure to not call memblock_reserve with a size of zero.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/setup.c

@@ -636,6 +636,8 @@ static void __init reserve_crashkernel(void)
 static void __init reserve_initrd(void)
 {
 #ifdef CONFIG_BLK_DEV_INITRD
+	if (!INITRD_START || !INITRD_SIZE)
+		return;
 	initrd_start = INITRD_START;
 	initrd_end = initrd_start + INITRD_SIZE;
 	memblock_reserve(INITRD_START, INITRD_SIZE);