h264: correct implicit weight table computation for long ref pics

Correct computation of implicit weight tables when referencing pictures
that are marked for long reference.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/h264.c

@@ -2157,15 +2157,17 @@ static void implicit_weight_table(H264Context *h, int field){
     for(ref0=ref_start; ref0 < ref_count0; ref0++){
         int poc0 = h->ref_list[0][ref0].poc;
         for(ref1=ref_start; ref1 < ref_count1; ref1++){
-            int poc1 = h->ref_list[1][ref1].poc;
-            int td = av_clip(poc1 - poc0, -128, 127);
-            int w= 32;
-            if(td){
-                int tb = av_clip(cur_poc - poc0, -128, 127);
-                int tx = (16384 + (FFABS(td) >> 1)) / td;
-                int dist_scale_factor = (tb*tx + 32) >> 8;
-                if(dist_scale_factor >= -64 && dist_scale_factor <= 128)
-                    w = 64 - dist_scale_factor;
+            int w = 32;
+            if (!h->ref_list[0][ref0].long_ref && !h->ref_list[1][ref1].long_ref) {
+                int poc1 = h->ref_list[1][ref1].poc;
+                int td = av_clip(poc1 - poc0, -128, 127);
+                if(td){
+                    int tb = av_clip(cur_poc - poc0, -128, 127);
+                    int tx = (16384 + (FFABS(td) >> 1)) / td;
+                    int dist_scale_factor = (tb*tx + 32) >> 8;
+                    if(dist_scale_factor >= -64 && dist_scale_factor <= 128)
+                        w = 64 - dist_scale_factor;
+                }
             }
             if(field<0){
                 h->implicit_weight[ref0][ref1][0]=