avformat/hlsenc: Fix leak of child AVFormatContext

Before ed897633, the hls muxer would free its child AVFormatContexts
and reset the pointer to these contexts to NULL immediately afterwards;
ed897633 moved the former to later (into a separate function), but kept
the resetting, ensuring that the child context leaks.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Steven Liu <lq@onvideo.cn>

libavformat/hlsenc.c

@@ -2631,7 +2631,6 @@ failed:
         }
         ffio_free_dyn_buf(&oc->pb);
 
-        vs->avf = NULL;
         av_free(old_filename);
     }