coap_time.h: cast COAP_TICKS_PER_SECOND to coap_tick_t for type safety
coap_time.h: cast COAP_TICKS_PER_SECOND to coap_tick_t for type safety

CID 1458396 detected by Coverity scan points out that integer
arithmetics may lead to an overflow without the typecast to
coap_tick_t.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap