avcodec: clarify documentation of avcodec_copy_context()

Following this is not strictly needed but safes some malloc+free
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -3284,7 +3284,7 @@ const AVClass *avcodec_get_subtitle_rect_class(void);
  * can use this AVCodecContext to decode/encode video/audio data.
  *
  * @param dest target codec context, should be initialized with
- *             avcodec_alloc_context3(), but otherwise uninitialized
+ *             avcodec_alloc_context3(NULL), but otherwise uninitialized
  * @param src source codec context
  * @return AVERROR() on error (e.g. memory allocation error), 0 on success
  */