x86: e820 memmap - add checking for NULL early param

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

arch/x86/kernel/e820.c

@@ -1198,6 +1198,9 @@ static int __init parse_memmap_opt(char *p)
 	char *oldp;
 	u64 start_at, mem_size;
 
+	if (!p)
+		return -EINVAL;
+
 	if (!strcmp(p, "exactmap")) {
 #ifdef CONFIG_CRASH_DUMP
 		/*