Merge "Replacing NOT_IMPLMENTED with NULL."

diff --cc vp8/vp8_cx_iface.c
Simple merge

diff --cc vp8/vp8_dx_iface.c
Simple merge

diff --cc vp9/vp9_cx_iface.c
Simple merge

diff --cc vp9/vp9_dx_iface.c
Simple merge

diff --cc vpx/internal/vpx_codec_internal.h
Simple merge