ffprobe: free shared writer context on exit

Fix leak.

Spotted-by: Michael Niedermayer

ffprobe.c

@@ -364,6 +364,7 @@ static void writer_close(WriterContext **wctx)
     if ((*wctx)->writer->priv_class)
         av_opt_free((*wctx)->priv);
     av_freep(&((*wctx)->priv));
+    av_opt_free(*wctx);
     av_freep(wctx);
 }