Clear err_code and err_reason at start of request handling

req->err_code and req->err_reason are set when going to synthetic
handling. From there the resp.reason HTTP field is set from
req->err_reason if set, or the generic code based on req->err_code is used
if it was NULL. This patch clears these members so that a value from the
handling of a previous request doesn't linger.

Fixes: VSV00004

bin/varnishd/cache/cache_req_fsm.c

@@ -802,6 +802,8 @@ cnt_recv_prep(struct req *req, const char *ci)
 	req->is_hit = 0;
 	req->is_hitmiss = 0;
 	req->is_hitpass = 0;
+	req->err_code = 0;
+	req->err_reason = NULL;
 }
 
 /*--------------------------------------------------------------------