Fix crash if invalid bit-rate was read from file.

Fixes issue 2426.

Patch by David Czech, davidczech510 gmail

Originally committed as revision 26061 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -1870,10 +1870,11 @@ static void av_estimate_timings_from_bit_rate(AVFormatContext *ic)
     AVStream *st;
 
     /* if bit_rate is already set, we believe it */
-    if (ic->bit_rate == 0) {
+    if (ic->bit_rate <= 0) {
         bit_rate = 0;
         for(i=0;i<ic->nb_streams;i++) {
             st = ic->streams[i];
+            if (st->codec->bit_rate > 0)
             bit_rate += st->codec->bit_rate;
         }
         ic->bit_rate = bit_rate;