avformat: prevent triggering request_probe assert in ff_read_packet

If probe_codec is called with pkt == NULL, it sets probe_packets to 0
and request_probe to -1.
However, request_probe can change when calling s->iformat->read_packet
and thus a probe_packets value of 0 doesn't guarantee a request_probe
value of -1.
In that case calling probe_codec again is necessary to prevent
triggering the assert.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/utils.c

@@ -803,7 +803,7 @@ int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
                 return ret;
             for (i = 0; i < s->nb_streams; i++) {
                 st = s->streams[i];
-                if (st->probe_packets)
+                if (st->probe_packets || st->request_probe > 0)
                     if ((err = probe_codec(s, st, NULL)) < 0)
                         return err;
                 av_assert0(st->request_probe <= 0);