avformat/mux: use only valid timestamps when calculating packet distance

Fixes: signed integer overflow: 0 - -9223372036854775808 cannot be represented in type 'long int'
Fixes: fate-cover-art-aiff-id3v2-remux, fate-cover-art-mp3-id3v2-remux and fate-mov-cover-image
under ubsan.

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/mux.c

@@ -951,6 +951,7 @@ int ff_interleave_packet_per_dts(AVFormatContext *s, AVPacket *pkt,
 
     if (s->max_interleave_delta > 0 &&
         si->packet_buffer.head &&
+        si->packet_buffer.head->pkt.dts != AV_NOPTS_VALUE &&
         !flush &&
         si->nb_interleaved_streams == stream_count+noninterleaved_count
     ) {