Fix crash with WPP when threads are disabled
Fix crash with WPP when threads are disabled

When WPP is enabled, a reference to SAO reconstruction job is copied
from the wavefront to the main encoder state. However, when threads are
disabled, the job is a null pointer and dereferencing it crashes the
encoder. Fixed by adding a null pointer check.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar