figure out new segment on the fly right in FrameDump::Start()

Source/Core/VideoCommon/FrameDump.cpp

@@ -84,9 +84,8 @@ bool FrameDump::Start(int w, int h)
 
   s_width = w;
   s_height = h;
-
-  s_last_frame_is_valid = false;
   s_last_pts = 0;
+  s_last_frame_is_valid = s_file_index != 0;
 
   InitAVCodec();
   bool success = CreateVideoFile();