nutenc: avoid negative timestamps

Fixes Ticket1468
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutenc.c

@@ -697,6 +697,9 @@ static int nut_write_header(AVFormatContext *s){
     if ((ret = write_headers(s, bc)) < 0)
         return ret;
 
+    if (s->avoid_negative_ts < 0)
+        s->avoid_negative_ts = 1;
+
     avio_flush(bc);
 
     return 0;