hwmon: (coretemp) Avoid leaving around dangling pointer

Storing the struct temp_data pointer allocated from create_core_data()
when returning an error has the potential of leaving around a pointer
to freed memory. Reset it to NULL for error returns.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>