Document and validate AVFrame plane pointers.

Check that the required plane pointers and only
those are set up.
Currently does not enforce anything for the palette
pointer of pseudopal formats as I am unsure about the
requirements.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavutil/frame.h

@@ -187,6 +187,9 @@ typedef struct AVFrame {
      * see avcodec_align_dimensions2(). Some filters and swscale can read
      * up to 16 bytes beyond the planes, if these filters are to be used,
      * then 16 extra bytes must be allocated.
+     *
+     * NOTE: Except for hwaccel formats, pointers not needed by the format
+     * MUST be set to NULL.
      */
     uint8_t *data[AV_NUM_DATA_POINTERS];