avformat/rtpenc_chain: use the proper function to free AVFormatContext

Fixes ticket #7075
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/rtpenc_chain.c

@@ -101,7 +101,7 @@ int ff_rtp_chain_mux_open(AVFormatContext **out, AVFormatContext *s,
     return 0;
 
 fail:
-    av_free(rtpctx);
+    avformat_free_context(rtpctx);
     if (handle)
         ffurl_close(handle);
     return ret;