fix stego_fprob to skip empty frame

conv/stego_fprob.cc

@@ -289,8 +289,11 @@ static void Exec() {
 
       const auto ret = dec->DecodeFrameNoDelay(&nal[i], sz, yuv, &frame);
       Enforce(ret == 0, "frame decode failure");
+      i += sz;
 
       Frame cf = {yuv, frame};
+      if (cf.w == 0 || cf.h == 0) continue;
+
       const auto utf = fidx%ut;
       if (utf > 0) {
         EachFrame(utf, cf, pf);
@@ -298,7 +301,6 @@ static void Exec() {
       pf = std::move(cf);
 
       ++fidx;
-      i += sz;
     }
   }
 }