With PVQ, the dst buffer should be initialized as zero
before av1_inv_txfm_add_*() is called.
This bug seems introduced during resolving conflicts
when nextgenv2 was merged.

BD-Rate change:
                PSNR  PSNR-HVS  SSIM  CIEDE 2000  MS SSIM
subset1-mono    -0.25 -0.25     -0.23 -0.26       -0.23
objective1-fast -0.17 -0.26     -0.14 -0.04       -0.18

Change-Id: I7c6b793ba0aa5f1e3d419312cbbe5c207a68f1f8