Merge commit '6cf27b55' into release/2.4

* commit '6cf27b55':
  aviobuf: check context before using it

See: 7441d1ecMerged-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/aviobuf.c

@@ -1098,7 +1098,6 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
         *pbuffer = NULL;
         return 0;
     }
-    d = s->opaque;
 
     /* don't attempt to pad fixed-size packet buffers */
     if (!s->max_packet_size) {
@@ -1108,6 +1107,7 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
 
     avio_flush(s);
 
+    d = s->opaque;
     *pbuffer = d->buffer;
     size = d->size;
     av_free(d);