avcodec/h264: Put context_count check back

Fixes assertion failure
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264.c

@@ -1270,10 +1270,12 @@ 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) {
-            ret = ff_h264_execute_decode_slices(h, context_count);
-            if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
-                goto end;
-            context_count = 0;
+            if (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. */