Assert that rate limit values are ok

src/rate_limit.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include "ucore/rate_limit.h"
 
 void uc_ratelimit_init(struct RateLimit *r, long tickets, long period, long current_ts)
@@ -7,6 +8,11 @@ void uc_ratelimit_init(struct RateLimit *r, long tickets, long period, long curr
     r->period = period;
     r->funds = r->ticket_cost * tickets;
     r->last_ts = current_ts;
+
+    assert(tickets > 0);
+    assert(period > 0);
+    assert(r->ticket_cost > 0);
+    assert(r->funds > 0);
 }
 
 void uc_ratelimit_reset(struct RateLimit *r, long current_ts)