don't dereference memory that has been freed (issue #6405)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@10139 65c4cc65-6c06-0410-ace0-fbb531ad65f3

file.c

@@ -720,11 +720,11 @@ int ast_closestream(struct ast_filestream *f)
 		free(f->realfilename);
 		f->realfilename = NULL;
 	}
-	f->fmt->close(f);
 	if (f->vfs) {
 		ast_closestream(f->vfs);
 		f->vfs = NULL;
 	}
+	f->fmt->close(f);
 	return 0;
 }