hwmon: (applesmc) Check key count before proceeding

commit 5f4513864304672e6ea9eac60583eeac32e679f2 upstream.

After reports from Chris and Josh Boyer of a rare crash in applesmc,
Guenter pointed at the initialization problem fixed below. The patch
has not been verified to fix the crash, but should be applied
regardless.

Reported-by: <jwboyer@fedoraproject.org>
Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>