Fix memory leak in libgsm wrapper.

Patch by Martin Storsjö, martin at martin dot st

Originally committed as revision 15798 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/libgsm.c

@@ -89,6 +89,7 @@ static av_cold int libgsm_init(AVCodecContext *avctx) {
 }
 
 static av_cold int libgsm_close(AVCodecContext *avctx) {
+    av_freep(&avctx->coded_frame);
     gsm_destroy(avctx->priv_data);
     avctx->priv_data = NULL;
     return 0;