avcodec/libaomdec: use intermediate arrays for plane pointers and strides

Fixes -Wstringop-overflow warnings with libaom >= 2.0.0, where the unused alpha
plane was removed from aom_image.

Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libaomdec.c

@@ -224,9 +224,13 @@ static int aom_decode(AVCodecContext *avctx, void *data, int *got_frame,
 
         if ((img->fmt & AOM_IMG_FMT_HIGHBITDEPTH) && img->bit_depth == 8)
             image_copy_16_to_8(picture, img);
-        else
-            av_image_copy(picture->data, picture->linesize, (const uint8_t **)img->planes,
-                          img->stride, avctx->pix_fmt, img->d_w, img->d_h);
+        else {
+            const uint8_t *planes[4] = { img->planes[0], img->planes[1], img->planes[2] };
+            const int      stride[4] = { img->stride[0], img->stride[1], img->stride[2] };
+
+            av_image_copy(picture->data, picture->linesize, planes,
+                          stride, avctx->pix_fmt, img->d_w, img->d_h);
+        }
         *got_frame = 1;
     }
     return avpkt->size;