x86: smpboot maxcpus - add checking for NULL early param

Signed-off-by: NCyrill Gorcunov <gorcunov@gmail.com>
Cc: akpm@linux-foundation.org
Cc: andi@firstfloor.org
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/smpboot.c

@@ -1452,7 +1452,8 @@ static int __init parse_maxcpus(char *arg)
 {
 	extern unsigned int maxcpus;
 
-	maxcpus = simple_strtoul(arg, NULL, 0);
+	if (arg)
+		maxcpus = simple_strtoul(arg, NULL, 0);
 	return 0;
 }
 early_param("maxcpus", parse_maxcpus);