Return correct decoded size, decoder is called with only one frame at a time.

patch by Benoit Fouet, benoit.fouet purplelabs com

Originally committed as revision 8633 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/amr.c

@@ -308,7 +308,7 @@ static int amr_nb_decode_frame(AVCodecContext * avctx,
     }
 
     //Each AMR-frame results in 160 16-bit samples
-    *data_size+=160*2;
+    *data_size=160*2;
 
     /* if not homed: check whether current frame is a homing frame */
     if (s->reset_flag_old == 0)