avformat/mpegts: Continue parsing PMTs during duration estimation

This way if we by chance run into a valid PMT we have a more complete
set of streams, also do not reset streams in case we run into a worse
PMT

Fixes Ticket4046

alternatively Ticket4046 could be closed as invalid or wontfix as it contains
some PMTs which lack the 2 subtitle streams
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -1803,10 +1803,10 @@ static void pmt_cb(MpegTSFilter *filter, const uint8_t *section, int section_len
 
     if (h->tid != PMT_TID)
         return;
-    if (ts->skip_changes)
-        return;
 
+    if (!ts->skip_clear)
         clear_program(ts, h->id);
+
     pcr_pid = get16(&p, p_end);
     if (pcr_pid < 0)
         return;