avcodec/h264_slice: clear frame only on gaps when it is not otherwise initilaized

Fixes: Timeout (53sec -> 31sec)
Fixes: 16908/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5711207859748864

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_slice.c

@@ -458,12 +458,6 @@ static int h264_frame_start(H264Context *h)
     H264Picture *pic;
     int i, ret;
     const int pixel_shift = h->pixel_shift;
-    int c[4] = {
-        1<<(h->ps.sps->bit_depth_luma-1),
-        1<<(h->ps.sps->bit_depth_chroma-1),
-        1<<(h->ps.sps->bit_depth_chroma-1),
-        -1
-    };
 
     if (!ff_thread_can_start_frame(h->avctx)) {
         av_log(h->avctx, AV_LOG_ERROR, "Attempt to start a frame outside SETUP state\n");
@@ -504,8 +498,6 @@ static int h264_frame_start(H264Context *h)
 
     if ((ret = alloc_picture(h, pic)) < 0)
         return ret;
-    if(!h->frame_recovered && !h->avctx->hwaccel)
-        ff_color_frame(pic->f, c);
 
     h->cur_pic_ptr = pic;
     ff_h264_unref_picture(h, &h->cur_pic);
@@ -1582,6 +1574,13 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
          * vectors.  Given we are concealing a lost frame, this probably
          * is not noticeable by comparison, but it should be fixed. */
         if (h->short_ref_count) {
+            int c[4] = {
+                1<<(h->ps.sps->bit_depth_luma-1),
+                1<<(h->ps.sps->bit_depth_chroma-1),
+                1<<(h->ps.sps->bit_depth_chroma-1),
+                -1
+            };
+
             if (prev &&
                 h->short_ref[0]->f->width == prev->f->width &&
                 h->short_ref[0]->f->height == prev->f->height &&
@@ -1597,7 +1596,8 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
                               prev->f->width,
                               prev->f->height);
                 h->short_ref[0]->poc = prev->poc + 2;
-            }
+            } else if (!h->frame_recovered && !h->avctx->hwaccel)
+                ff_color_frame(h->short_ref[0]->f, c);
             h->short_ref[0]->frame_num = h->poc.prev_frame_num;
         }
     }