hwmon: (applesmc) Check key count before proceeding

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>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>