avformat/hlsenc: fix memleak in hls_write_trailer

fix CID: 1426931
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/hlsenc.c

@@ -2538,6 +2538,7 @@ static int hls_write_trailer(struct AVFormatContext *s)
             filename = av_asprintf("%s", vs->avf->url);
         }
         if (!filename) {
+            av_free(old_filename);
             return AVERROR(ENOMEM);
         }