lavu/imgutils: add consistency checks to av_image_copy_plane()

Add assertions and abort in case of invalid |dst_linesize| < bytewidth or
|src_linesize| < bytewidth.

Avoid to silently corrupt memory.

libavutil/imgutils.h

@@ -99,6 +99,9 @@ int av_image_alloc(uint8_t *pointers[4], int linesizes[4],
  * The first byte of each successive line is separated by *_linesize
  * bytes.
  *
+ * bytewidth must be contained by both absolute values of dst_linesize
+ * and src_linesize, otherwise the function behavior is undefined.
+ *
  * @param dst_linesize linesize for the image plane in dst
  * @param src_linesize linesize for the image plane in src
  */