rtpenc_mpegts: Set chain->rtp_ctx only after avformat_write_header succeeded

By making sure we at each time only have one pointer set, either a
local variable or one in the context, we avoid potential double frees
in the cleanup routines. If chain->rtp_ctx is set, it is closed by
calling avformat_write_trailer, but that shouldn't be called unless
avformat_write_header succeeded.

This issue was pointed out by Andreas Cadhalpun.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc_mpegts.c

@@ -88,11 +88,10 @@ static int rtp_mpegts_write_header(AVFormatContext *s)
     st->time_base.num   = 1;
     st->time_base.den   = 90000;
     st->codec->codec_id = AV_CODEC_ID_MPEG2TS;
-    chain->rtp_ctx = rtp_ctx;
     rtp_ctx->pb = s->pb;
     if ((ret = avformat_write_header(rtp_ctx, NULL)) < 0)
         goto fail;
-    rtp_ctx = NULL;
+    chain->rtp_ctx = rtp_ctx;
 
     return 0;