s390/vmcp: correct early_param handling

Check "p" is not NULL before passing it to memparse, which doesn't
handle that case explicitly.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/char/vmcp.c

@@ -43,6 +43,8 @@ static struct cma *vmcp_cma;
 
 static int __init early_parse_vmcp_cma(char *p)
 {
+	if (!p)
+		return 1;
 	vmcp_cma_size = ALIGN(memparse(p, NULL), PAGE_SIZE);
 	return 0;
 }