scan-build: in cpuworker, initialize tv_start

scan-build doesn't realize that a request can't be timed at the end
unless it's timed at the start, and so it's not possible for us to
be subtracting start from end without start being set.
Nevertheless, let's not confuse it.

src/or/cpuworker.c

@@ -436,7 +436,7 @@ cpuworker_main(void *data)
     if (req.task == CPUWORKER_TASK_ONION) {
       const create_cell_t *cc = &req.create_cell;
       created_cell_t *cell_out = &rpl.created_cell;
-      struct timeval tv_start, tv_end;
+      struct timeval tv_start = {0,0}, tv_end;
       int n;
       rpl.timed = req.timed;
       rpl.started_at = req.started_at;