fix memleak, based upon a patch by Wolfram Gloger

Originally committed as revision 4947 to svn://svn.ffmpeg.org/ffmpeg/trunk

output_example.c

@@ -512,6 +512,7 @@ int main(int argc, char **argv)
 
     /* free the streams */
     for(i = 0; i < oc->nb_streams; i++) {
+        av_freep(&oc->streams[i]->codec);
         av_freep(&oc->streams[i]);
     }