ACPI: processor: idle: Check acpi_fetch_acpi_dev() return value

commit 2437513a814b3e93bd02879740a8a06e52e2cf7d upstream.

The return value of acpi_fetch_acpi_dev() could be NULL, which would
cause a NULL pointer dereference to occur in acpi_device_hid().

Signed-off-by: Li Zhong <floridsleeves@gmail.com>
[ rjw: Subject and changelog edits, added empty line after if () ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Signed-off-by: Yann Sionneau <yann.sionneau@vates.tech>
Reported-by: Dillon C <dchan@dchan.tech>
Tested-by: Dillon C <dchan@dchan.tech>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>