ACPI: detect invalid argument written to /proc/acpi/alarm

/proc/acpi/alarm can't be set correctly, here is a sample:

[root@localhost /]# echo "2006 09" > /proc/acpi/alarm
[root@localhost /]# cat /proc/acpi/alarm
2007-12-09 09:09:09
[root@localhost /]# echo "2006 04" > /proc/acpi/alarm
[root@localhost /]# cat /proc/acpi/alarm
2007-12-04 04:04:04
[root@localhost /]#

Obviously, it is wrong, it should consider it as an invalid input.

after this patch:

[root@localhost /]# echo "2008 09" > /proc/acpi/alarm
-bash: echo: write error: Invalid argument
[root@localhost /]#
Signed-off-by: NYi Yang <yi.y.yang@intel.com>
Signed-off-by: NLen Brown <len.brown@intel.com>

drivers/acpi/sleep/proc.c

@@ -178,6 +178,9 @@ static int get_date_field(char **p, u32 * value)
 	 * Try to find delimeter, only to insert null.  The end of the
 	 * string won't have one, but is still valid.
 	 */
+	if (*p == NULL)
+		return result;
+
 	next = strpbrk(*p, "- :");
 	if (next)
 		*next++ = '\0';
@@ -190,6 +193,8 @@ static int get_date_field(char **p, u32 * value)
 
 	if (next)
 		*p = next;
+	else
+		*p = NULL;
 
 	return result;
 }