Fix formatting for negative start times (issue 2139).

Patch by Christian d'Heureuse, chdh inventec ch

Originally committed as revision 25063 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -3135,7 +3135,7 @@ void dump_format(AVFormatContext *ic,
             int secs, us;
             av_log(NULL, AV_LOG_INFO, ", start: ");
             secs = ic->start_time / AV_TIME_BASE;
-            us = ic->start_time % AV_TIME_BASE;
+            us = abs(ic->start_time % AV_TIME_BASE);
             av_log(NULL, AV_LOG_INFO, "%d.%06d",
                    secs, (int)av_rescale(us, 1000000, AV_TIME_BASE));
         }