Backport PR #3813 to 2.4

It has already been partially backported by #6199; this commit completes
the backport.

modules/highgui/src/cap_ffmpeg_impl.hpp

@@ -853,8 +853,6 @@ bool CvCapture_FFMPEG::grabFrame()
         frame_number > ic->streams[video_stream]->nb_frames )
         return false;
 
-    av_free_packet (&packet);
-
     picture_pts = AV_NOPTS_VALUE_;
 
     // get the next frame
@@ -917,8 +915,6 @@ bool CvCapture_FFMPEG::grabFrame()
             if (count_errs > max_number_of_attempts)
                 break;
         }
-
-        av_free_packet (&packet);
     }
 
     if( valid && first_frame_number < 0 )