do not display par and dar if not available

Originally committed as revision 12355 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -1128,6 +1128,7 @@ void avcodec_string(char *buf, int buf_size, AVCodecContext *enc, int encode)
             snprintf(buf + strlen(buf), buf_size - strlen(buf),
                      ", %dx%d",
                      enc->width, enc->height);
+            if (enc->sample_aspect_ratio.num) {
             av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
                       enc->width*enc->sample_aspect_ratio.num,
                       enc->height*enc->sample_aspect_ratio.den,
@@ -1136,6 +1137,7 @@ void avcodec_string(char *buf, int buf_size, AVCodecContext *enc, int encode)
                      " [PAR %d:%d DAR %d:%d]",
                      enc->sample_aspect_ratio.num, enc->sample_aspect_ratio.den,
                      display_aspect_ratio.num, display_aspect_ratio.den);
+            }
             if(av_log_get_level() >= AV_LOG_DEBUG){
                 int g= ff_gcd(enc->time_base.num, enc->time_base.den);
                 snprintf(buf + strlen(buf), buf_size - strlen(buf),