mpegts: fix null-pointer dereference

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -1125,7 +1125,7 @@ static void pmt_cb(MpegTSFilter *filter, const uint8_t *section, int section_len
             pes = ts->pids[pid]->u.pes_filter.opaque;
             if (!pes->st) {
                 pes->st = avformat_new_stream(pes->stream, NULL);
-                st->id = pes->pid;
+                pes->st->id = pes->pid;
             }
             st = pes->st;
         } else {