avfilter/vf_phase: remove dead initialization

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avfilter/vf_drawtext: remove dead initialization

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avcodec/dpx: return different error code for unsupported depths

Also give better message in such cases.

Signed-off-by: Paul B Mahol <onemda@gmail.com>