This will fix Assertion in get_txb_ctx() for invalid sign value

After fixing this bug, we have around 0.2% gain on lowres.

With cb4x4 on only, lv_map provides 1.538% gain on lowres.

With cb4x4 + chroma_sub8x8, lv_map provides 1.468% gain on lowres.

Change-Id: I06f996ec5af0d5e79e377a3dc8c012862fc4b9c7