avformat/utils: Clear pointer in ff_alloc_extradata() to avoid leaving a stale pointer in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit bbfca8e8)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2829,6 +2829,7 @@ int ff_alloc_extradata(AVCodecContext *avctx, int size)
     int ret;
 
     if (size < 0 || size >= INT32_MAX - FF_INPUT_BUFFER_PADDING_SIZE) {
+        avctx->extradata = NULL;
         avctx->extradata_size = 0;
         return AVERROR(EINVAL);
     }