riff: Fix potential memleak.

Make ff_get_wav_header() free existing extradata before allocing a new
buffer.

libavformat/riff.c

@@ -509,6 +509,7 @@ int ff_get_wav_header(AVIOContext *pb, AVCodecContext *codec, int size)
         }
         codec->extradata_size = cbSize;
         if (cbSize > 0) {
+            av_free(codec->extradata);
             codec->extradata = av_mallocz(codec->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
             if (!codec->extradata)
                 return AVERROR(ENOMEM);