ffmpeg: factorize quality calculation

Calculate quality value once per stream in print_report().
Also fix segfault, as coded_frame can be NULL.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

ffmpeg.c

@@ -1322,26 +1322,25 @@ static void print_report(AVFormatContext **output_files,
     ti1 = 1e10;
     vid = 0;
     for(i=0;i<nb_ostreams;i++) {
+        float q = -1;
         ost = ost_table[i];
         enc = ost->st->codec;
+        if (!ost->st->stream_copy && enc->coded_frame)
+            q = enc->coded_frame->quality/(float)FF_QP2LAMBDA;
         if (vid && enc->codec_type == AVMEDIA_TYPE_VIDEO) {
-            snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "q=%2.1f ",
-                     !ost->st->stream_copy ?
-                     enc->coded_frame->quality/(float)FF_QP2LAMBDA : -1);
+            snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "q=%2.1f ", q);
         }
         if (!vid && enc->codec_type == AVMEDIA_TYPE_VIDEO) {
             float t = (av_gettime()-timer_start) / 1000000.0;
 
             frame_number = ost->frame_number;
             snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "frame=%5d fps=%3d q=%3.1f ",
-                     frame_number, (t>1)?(int)(frame_number/t+0.5) : 0,
-                     !ost->st->stream_copy ?
-                     enc->coded_frame->quality/(float)FF_QP2LAMBDA : -1);
+                     frame_number, (t>1)?(int)(frame_number/t+0.5) : 0, q);
             if(is_last_report)
                 snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "L");
             if(qp_hist){
                 int j;
-                int qp= lrintf(enc->coded_frame->quality/(float)FF_QP2LAMBDA);
+                int qp = lrintf(q);
                 if(qp>=0 && qp<FF_ARRAY_ELEMS(qp_histogram))
                     qp_histogram[qp]++;
                 for(j=0; j<32; j++)