avcodec/h264: Fix off by 1 context count

Fixes fate-h264-xavc-4389 with slice threads

libavcodec/h264.c

@@ -1270,12 +1270,10 @@ again:
                 av_log(h->avctx, AV_LOG_ERROR, "decode_slice_header error\n");
             sl->ref_count[0] = sl->ref_count[1] = sl->list_count = 0;
         } else if (err == SLICE_SINGLETHREAD) {
-            if (context_count > 1) {
-                ret = ff_h264_execute_decode_slices(h, context_count - 1);
-                if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
-                    goto end;
-                context_count = 0;
-            }
+            ret = ff_h264_execute_decode_slices(h, context_count);
+            if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
+                goto end;
+            context_count = 0;
             /* Slice could not be decoded in parallel mode, restart. Note
              * that rbsp_buffer is not transferred, but since we no longer
              * run in parallel mode this should not be an issue. */