avcodec/mpegaudiodec_template: use av_freep(), do not leave stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegaudiodec_template.c

@@ -1819,7 +1819,7 @@ static av_cold int decode_close_mp3on4(AVCodecContext * avctx)
     int i;
 
     for (i = 0; i < s->frames; i++)
-        av_free(s->mp3decctx[i]);
+        av_freep(&s->mp3decctx[i]);
 
     return 0;
 }