Don't fill in frame gaps with copied refs after flush

The filled in refs cause corruptions in the video frame
for a long time after it should have recovered.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -2367,7 +2367,7 @@ static void implicit_weight_table(H264Context *h, int field){
 static void idr(H264Context *h){
     int i;
     ff_h264_remove_all_refs(h);
-    h->prev_frame_num= 0;
+    h->prev_frame_num= -1;
     h->prev_frame_num_offset= 0;
     h->prev_poc_msb=
     h->prev_poc_lsb= 0;
@@ -2882,7 +2882,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
 
     if(h0->current_slice == 0){
         // Shorten frame num gaps so we don't have to allocate reference frames just to throw them away
-        if(h->frame_num != h->prev_frame_num) {
+        if(h->frame_num != h->prev_frame_num && h->prev_frame_num >= 0) {
             int unwrap_prev_frame_num = h->prev_frame_num, max_frame_num = 1<<h->sps.log2_max_frame_num;
 
             if (unwrap_prev_frame_num > h->frame_num) unwrap_prev_frame_num -= max_frame_num;
@@ -2896,7 +2896,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
             }
         }
 
-        while(h->frame_num !=  h->prev_frame_num &&
+        while(h->frame_num !=  h->prev_frame_num && h->prev_frame_num >= 0 &&
               h->frame_num != (h->prev_frame_num+1)%(1<<h->sps.log2_max_frame_num)){
             Picture *prev = h->short_ref_count ? h->short_ref[0] : NULL;
             av_log(h->s.avctx, AV_LOG_DEBUG, "Frame num gap %d %d\n", h->frame_num, h->prev_frame_num);