xlog: fix a memory leak in xlog_write_meta()

Free the vclock_to_string() buffer in all cases.

src/box/xlog.cc

@@ -729,7 +729,7 @@ xlog_write_meta(struct xlog *l)
 		free(vstr);
 		return -1;
 	}
-
+	free(vstr);
 	return 0;
 }