avformat/oggdec: reset end_trimming when it has been used, so it cannot be used twice by mistake

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggdec.c

@@ -785,6 +785,7 @@ retry:
             return AVERROR(ENOMEM);
         }
         AV_WL32(side_data + 4, os->end_trimming);
+        os->end_trimming = 0;
     }
 
     return psize;