mpegts: Support running the write_trailer function without an AVIOContext

If opening and closing dynamic buffers as AVIOContext, we may
not have any AVIOContext available when wanting to close and
deallocate the muxer. Allow calling write_trailer despite this.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/mpegtsenc.c

@@ -1192,7 +1192,8 @@ static int mpegts_write_end(AVFormatContext *s)
     MpegTSService *service;
     int i;
 
-    mpegts_write_flush(s);
+    if (s->pb)
+        mpegts_write_flush(s);
 
     for (i = 0; i < s->nb_streams; i++) {
         AVStream *st = s->streams[i];